etymology - Why is たばこ written in hiragana instead of katakana?

While studying my Genki textbook I noticed that the word たばこ uses hiragana instead of katakana. I've always seen loan words to be in katakana, so this was interesting to me. I saw in this answer that this is because its an older word. I was wanting to know if there was more information about this. Why is たばこ, a loan word, written in hiragana?

Answer

The simplest explanation is that たばこ is often written in hiragana because it was borrowed so long ago that it has nativized. (Sorry, in a rush -- details to be posted later.)

Update

There are a few words that are definitely borrowings, but that were borrowed so long ago that they are treated as native terms, and they might even have kanji spellings.

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  Tabako is one such word. The substance was introduced to Japan by the Spanish and Portuguese missionaries in the late 1500s. The source of the Japanese term is generally listed as Portuguese tabaco. The term may be encountered in hiragana as たばこ, in katakana as タバコ, and in kanji as 煙草 (most common, even offered up by the MS IME), or as 烟草 or 莨 (less common, listed in some dictionaries).

  
  


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  Tenpura is another well-known Japanese term that was borrowed. This too has kanji spellings, as 天麩羅 or 天婦羅, even though the term again comes from Portuguese. See the Wiktionary entry for more detail.

  
  

As a general rule of thumb, borrowings are indeed spelled in katakana. However, the older the borrowing, the less this rule applies. For instance, this other question-and-answer thread lists several old borrowings from Korean that are treated as nativized Japanese words, complete with kanji spellings. For newer borrowings, especially from English, katakana is the way to go.

Update 2

More details about the history of the term タバコ, with references (emphasis and translations mine).

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  Shogakukan's 国語大辞典 mentions the following:

  
  

  
  

  原産地では古代から吸煙に利用され、一六世紀初頭にスペインに伝わり、世界じゅうに急速に普及、日本へは安土桃山時代に輸入された。
  Used in its place of origin since ancient times for smoking, reaching Spain in the early 1500s, rapidly spreading around the world, imported to Japan in the Azuchi-Momoyama period.

  
  
  

  
  

  The 安土{あづち}桃山{ももやま}時代{じだい} (Azuchi-Momoyama period) dates from 1573 to 1603.

  
  


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  Daijirin explains:

  
  

  
  

  日本へは近世初頭に南蛮船によってもたらされた。
  Brought to Japan by ships from Spain or Portugal in the early modern ages.

  
  

  
  

  近世{きんせい} ("early modern ages") is further defined more specifically as:

  
  
  

  
  

  日本史では，後期封建制の時期の安土桃山・江戸時代をいう。
  In Japanese history, refers to the late-feudal Azuchi-Momoyama and Edo periods.

  
  

  
  


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  Daijisen explains:

  
  

  
  

  南アメリカの原産で、桃山時代に輸入。
  Originally from South America, imported in the Momoyama period.

  
  
  

  
  


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  Gogen Allguide explains:

  
  

  
  

  日本には南蛮貿易を通して伝わり、慶長年間（1596～1615年）​にはタバコの栽培が行われた。
  Reached Japan via trading with the Spanish and Portuguese, with tobacco then cultivated in the Keichō era (1596-1615).

  
  

  
  

  Cultivation was preceded by importation of the product and the term, and this timing again coincides with the other sources.