Why are the servers down so often?

[DaC]

There's been frequent maintenance because good players have been acting in good faith by reporting bugs and exploits so Square can fix them. Case in point, certain abused Fates that people farmed ad nauseaum.

If you are referring to things like Dark Devices fate. It wasn't an exploit according to a post by a CM. Although they did adjust it soon after he made that post.