Why are "loanwords" written in katakana?


I am interested in learning why katakana was chosen to represent foreign
words.




Because that's the original purpose of katakana. Katakana was invented to
denote the readings of unfamiliar foreign words (although, in those days,
"foreign words" mainly referred to words in Asian languages such as Sanskrit,
Pali, Chinese). On the other hand, hiragana developed as the shorthand for
native Japanese sentences written in Manyo-gana. For details, please read the
following questions:

  * [Why was both katakana and hiragana created?](https://japanese.stackexchange.com/q/4281/5010)
  * [Why are there 3 ways of writing in Japanese?](https://japanese.stackexchange.com/q/3984/5010)

The Japanese writing system underwent multiple major overhauls in the last
1500 years, but one main role of katakana basically remained the same: to
serve as the phonetic alphabet to purely represent the sounds of non-ordinary
words.




Because that's the original purpose of katakana. Katakana was invented to
denote the readings of unfamiliar foreign words (although, in those days,
"foreign words" mainly referred to words in Asian languages such as Sanskrit,
Pali, Chinese). On the other hand, hiragana developed as the shorthand for
native Japanese sentences written in Manyo-gana. For details, please read the
following questions:

  * [Why was both katakana and hiragana created?](https://japanese.stackexchange.com/q/4281/5010)
  * [Why are there 3 ways of writing in Japanese?](https://japanese.stackexchange.com/q/3984/5010)

The Japanese writing system underwent multiple major overhauls in the last
1500 years, but one main role of katakana basically remained the same: to
serve as the phonetic alphabet to purely represent the sounds of non-ordinary
words.

