A combination of the people, my real life activities and my personal state of mind.
I just found that Avalon wasn't worth the effort any more. For the amount of energy and hours I was putting in, I was getting very little out of it. Investing that much of myself into something that I wasn't enjoying just wasn't healthy for me. The things that made the game worthwhile for me - having fun and interacting with my game buddies just weren't there anymore. It's probably more my own outlook on life changing than anything in Avalon changing.
It got to the stage where I was just disgusted with myself for interacting purely with text for 50+ hours a week and just logged out and deleted all my Avalon stuff then and there.
What keeps me from returning to the game is that I found something else to do with my time - unlike previous occasions where I've returned purely cause I have nothing else to do. I've managed to fill the chuck of time that was dedicated to Avalon with other stuff, rather than just sitting on my ass doing nothing. I'm doing something every day after school every week, so I simply don't have the time for Avalon anymore.
All in all, I think the most important factor is that I started playing Avalon at a very shit time in my life - going through the usual high school dramas and such. It was like a form of escapism for me - now that I've sorted myself out, I don't really need that escape anymore, thus Avalon became redundant. It was a chore to log in, instead of being a relief - it was like a job that I wasn't getting paid for. Screw that!