One of the problems is that hashtag usage evolved into a form of signaling for emphasis (#your-slogan-here) instead of a useful way to tag a topic, to the extent that it can read as obnoxious if you do it at all in a serious way.

Another is that it's another moderation headache. Anyone can insert a hashtag for any reason, so anyone will. If you don't police it, it becomes useless; if you do police it, it becomes heavy.