Why was "アオ" used instead of ”あお”?

Question

I was reading a picture book, and there was this sentence: アオくて　ちいさな　ぼくのイス。

Why is "ao" and "isu" written in katakana and not hiragana?

Answer 1

It's done because it makes it easier to read and understand when no kanji are used.

Easy to read (general usage)

[青]{あお}くて[小]{ちい}さな[僕]{ぼく}の[椅子]{いす}。

Normal(this question)

アオくて　ちいさな　ぼくのイス。

Hard to read (hard to understand)

あおくて　ちいさな　ぼくのいす。

Hardest to read (this might not be understandable)

あおくてちいさなぼくのいす。

Impossible to read (joke :-P)

あおくてち　いさなぼ　くのいす。