Because it conflates distinct characters and therefore fails its original mission to encode all characters needed for written communication.

Han Unification is just the most obvious case but the issues do not stop there. I'll give you a western example. In the sentence

«Günther a souligné l’ambigüité de son discours.»

there is an umlaut and a dieresis.

They are different things with different function. In traditional book printing they used to look differently.

With Unicode all this cultural nuance is lost. The characters necessary to communicate precisely simply have never been encoded, because Unicode forgot about its core mission.

Fixing things like that is where I want to see efforts go.