orthography - Why was "アオ" used instead of ”あお”?

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

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

[青]{あお}くて[小]{ちい}さな[僕]{ぼく}の[椅子]{いす}。

アオくて　ちいさな　ぼくのイス。

あおくて　ちいさな　ぼくのいす。

あおくてちいさなぼくのいす。

あおくてち　いさなぼ　くのいす。