1. "We need a better image host!"
2. Makes new image host.
3. Uh oh, hosting costs money.
4. Ads.
5. Lawsuits because illegal stuff. Uh oh, lawyers cost money.
6. "We need a better image host!"
This is a really important point. From what I can see, Imgur doesn't have a porn community, it just has a lot of porn because it's used as an image host by outside communities. Porn is probably a net drain on profit for them, since they make their revenue off of people who use Imgur itself, not from servicing direct image links posted on Reddit.
To me it seems like it's not necessarily budgeting issue but natural greed kicking in. I'm sure imgur people are not tightening their belts over this.