It's been pretty successful usage wise, but paying users are another matter entirely. Donations and account upgrades keep the lights on, barely covering the hosting costs of the main collocated front-end server, but lack of funds is a constant struggle. I've spent tens of thousands over the years on hardware and hosting costs, and expenses keep going up as coverage expands. As for why, I guess there's just not enough of a reason to upgrade at the moment. Free is hard to beat, I'm my own best competitor.
The site's current design is basic to say the least, and the account creation page does not leave users feeling especially comfortable about the site. Everything about everything needs polish.
Related to that, I'm almost finished with a redesigned and much nicer looking front-end, but that probably won't magically solve all my problems. ;)