There are two reasons between this place having been dying.
The first is that a mass deletion accident occurred a couple of years ago. Some guy got ahold of a staffer's password, hijacked their account and used it to delete a shit-ton of pics. Since Booru's system is also such that you can't upload an identical pic of one's that's been deleted, it also made it much harder to restore what had been lost. I think we lost over a hundred pics and many still haven't been restored.
After that, I saw a lot less activity here.
The second was that some guy created a bot that he used to downvote a lot of pics; 'twas about a year ago, I think. The bot downvoted a lot of pictures down to at least -3, though usually around -7. This probably turned a lot of uploaders off because why keep uploading stuff if it's just gonna get downvoted and hidden?
Because of this, staff had to block new user registration, so no new users could join (dunno of it's up again). This means that we keep losing people while no new ones are coming to replace them.
So yeah, like Purple said: trolls and bots are what's destroying it.
Lately though, it seems that activity has been picking up again, so maybe it's not dying anymore.