The odd katakana words I discuss here fall into three related categories:
1) common words where a katakana loan word is used when a non-loan word is available
2) place names in China, when katakana is used but readable kanji is available
3) words written in hiragana or kanji that would make better sense as katakana loan words
1. There are quite a number of cases when a katakana loan word is used to express something that could be expressed without resort to loan words. For example, “cancer” can be written as kanji (癌, pronounced がん) or as katakana (ガン).1 Apparently there are a large number of scientific words written in katakana, though not all of them utilize both kanji and katakana.2 This makes a great deal of sense historically; when Japan was opening up to Europe and America in the latter half of the 1800s, scientific knowledge was one of the things most sought-after by Japan. Certainly, knowledge of Western science and technology would set Japan apart from its neighbors and help prevent it from sharing the fate China suffered under the Unequal Treaty (不平等条約；ふびょうどうじょうやく).3 Since science, or at least this manifestation of science, was thought to come from the West, it makes sense that Western words are often used for scientific ideas. Still, it makes an interesting clash when those loan words either co-exist with non-loan words that express the same meaning.
Another example of a loan word used when a non-loan word would do is トイレ.4 There are several other words that can mean “toilet”: お手洗い and 便所（べんじょ）to name just two.5 There are a couple possible reasons トイレ might have been added to the Japanese lexicon. The first is that it may have simply been added as an additional word to refer to toilets; many languages, including English, have a multitude of words to name the place one goes to relieve oneself. In this case, it would have been viewed as synonymous with 便所. On the other hand, トイレ may have come into use specifically to delineate a perceived difference between Western and traditional Japanese toilets. Either Westerners may have insisted that the two are not the same, or Japanese may have come to that conclusion themselves. The perceived difference would likely have centered around sitting versus squatting toilets. Unfortunately, I have not been able to find out when トイレ came into use and who introduced it.
2. Quite a number of Chinese place names can be written either in kanji or in katakana. In the case of Hong Kong, it almost makes more sense to call it ホンコン than 香港—after all, the city was held by Britain for a hundred years and is best known to the world as “Hong Kong”, not the modern Mandarin pronunciation “Xianggang”.6 But with other cities, this makes less sense. Beijing can be written ペキン as well as 北京; Nanjing can be written ナンキン as well as 南京; Shanghai can be written シャンハイ as well as 上海 (though that one can apparently also be written in hiragana, at least according to WWWJDIC online dictionary).7 For pronunciation purposes alone, the katakana may be used simply to remind the Japanese audience to eschew the normal pronunciations of these characters. Yet this reasoning is not entirely satisfactory: Japanese is already accustomed to shifting pronunciation based on what combinations a character is located in, so it should not be too difficult to remember the peculiar combinations arising from Chinese place names. Also, if pronunciation aid were the only reason to use katakana, that katakana should be used in furigana (the small kana writing above or below kanji), rather than in place of kanji. When the katakana is used in place of the kanji, it suggests to me a desire on the part of Japan to distance itself from China. By eschewing their shared characters and using katakana, the Japanese language treats China as simply another foreign country, rather than one with whom it has a long and intimate history. Given the tensions that have frequently plagued Sino-Japanese relations, it makes sense that there would be times Japan would wish to distance itself from China. Similarly, the kanji may be used when the writer wishes to stress a commonality with one of these cities, such as in a comparison between Asia and the West (whatever is meant by West). While I cannot guarantee this is the reasoning for using katakana to write place names that already have kanji assigned, it seems plausible.
3. In opposition to the first point, there are some words that are not written in katakana, yet based on their meaning, it seems that they should be. たばこ is one; 野球（やきゅう）is another.8 Neither one is native to Japan (tobacco certainly isn’t, and seems to have been introduced to Japan by the Dutch and Portuguese, and I very much doubt Japan had any game similar enough to baseball to warrant using the same name). Yet, contrary to the norm for foreign words, these have not been given katakana names. Actually, apparently “tobacco” can be written either たばこ or タバコ.9 But in the case of baseball, not only is the word written in hiragana, it has also acquired kanji. And baseball is a particularly odd case, since the majority of other sports are written with katakana. I have no idea why these words are not written in katakana, and find it most baffling.
It is possible that Japanese felt a particular affinity for things such as 野球 and 冷蔵庫（れいぞうこ）, another example, and so internalized them by writing them in hiragana and giving them kanji.10 Although I have not heard of a strong fascination in Japan with baseball, and while refrigerators are popular everywhere, I again have not heard anything to indicate that Japan has embraced them to a noteworthy degree (vending machines, on the other hand...). Also, 冷蔵庫 has its own twist; unlike 野球, this is a more-or-less direct translation of the earlier English name for refrigerators: ice box. The Chinese name for refrigerators, 冰箱, is also a direct translation of this early English name; I don't know if the Chinese name influenced Japan's decision how to render this word or not (though it should be noted the words are not identical). But these possible explanations do not extend to 野球; this is neither a translation of the English name or similar to the Chinese name (棒球）. As an overall category, I am still deeply confused why some clearly foreign-origin words have been rendered in hiragana and often given kanji.
Overall, it is possible that, as Woodlandcreature comments, the volume of foreign words entering Japan may relate to whether the words were left as loan words or not. I would expect this would be a direct relationship, with greater volumes of foreign words entering Japan leading to greater numbers of words left as katakana loan words. Determining this would require a close analysis of when katakana loan words originated and the levels of Japan’s international involvement at those times.
Yet this theory still does not address why katakana loan words have been adopted for words that already had a Japanese term, such as トイレ. Woodlandcreature him/herself points out another example in this category: camera, which after being in the West, went from being called 写真機（しゃしんき）to the now-popular term カメラ.
While I think Woodlandcreature’s theory may well be at least partially correct, I think a greater contributing factor may be the innate conservative nature of Japanese. When I spoke with a friend who has been studying Japanese for many years, she claimed that Japan has an extremely conservative language (and culture); she jokingly said that anything, from Elvis to the Kentucky Fried Chicken colonel, may die in the rest of the world, but it will live on in some fashion in Japan. (In the US, the restaurant founded by Sanders now goes by KFC and is colonel-less; apparently that is not the case in Japan.) To bring this back to language, while it may seem odd to link the wholesale adoption of a multitude of foreign words with linguistic conservatism, I think the two are quite likely related. Adopting foreign words wholesale can be seen as a way to keep these foreign words away from the “pure” or “authentic” Japanese words (many of which were also borrowed, just much earlier). Japanese is even able to put these foreign words in a separate script, a script that is mostly used for these foreign words, so Japanese readers are usually able to tell instantly if a word is a foreign loan word or not. As Huixin so eloquently puts it, there seems to be “a constant tension between trying to ‘nativize’ foreign words by appropriating them within the Japanese lexicon and retaining the foreign feeling of non-native words by writing them in katakana”. I may here be ascribing intentions to developers of recent Japanese words that do not exist. On the other hand, the conservatism of the Japanese language is hard to dispute.
4. Minna no Nihongo I grammar textbook, chapter 3
6. Minna no Nihongo I grammar textbook, chapter 12
8. Minna no Nihongo I grammar textbook, chapters 3 and 9
