Why is という in ボソニックという食べ物 (for example) written in ひらがな rather than と言う? In my understanding, ボソニックという食べ物 means "a food called Bosonic".
In this usage, という is almost an independent particle "that, called, known as" rather than the combination of と + 言う, thus no longer any point to retain original kanji (that symbolizes the "real" meaning). It's just like we spell English words anyway, into or nevertheless in one word for the specific meaning.
That said, it's not a hard rule that you must change kanji usage in this manner. Some people would write 言う all the time, while others would always use いう. But it could be said that it's a "better practice".
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