1. The vocabulary in hiragana is larger than the vocabulary in kanji. An author cannot always assume a reader knows the appropriate kanji, aside from the core 1200 (or whatever; basic adult readership) or the ~5000 that the average adult reader knows. Rant, rant, rant.

2. Even some words written in kanji used hiragana for modifiers, verb endings, etc.

3. Grammatical elements (conjunctions, particles, etc) are written in hiragana.