I think the main reason was due to kid safety concerns (being able to draw you could post anything including profanities, sexual content etc) and people being able to post any image they wanted through the use of homebrew raised additional concerns. You know how Nintendo is with kid safety, not allowing any form of chat in their games and often not even allowing any form of chat between friends in the actual console firmware. Anything that can be abused is out of the question and Miiverse was abused on many occasions despite their heavy moderation of it, I guess they just gave up on trying.