I've left both Eve Online and Vendetta Online behind for the same main reason. I get very addicted to games, with everything else suffering ie friends, girlfriend, work etc. Now this is alright when its say GTA because eventually you finish it and get back to the real world, but MMOs just never end. Basically I find them incompatible with have a well balanced life.
Next is the money, if I played an MMO for a year I'd have paid somewhere in 3 figures, that's just too much for a game.
Finally, to get what I want out of them you have to be far more serious about games than I'm willing to be. By that I mean you have to go on at specific times to do things. Now for me any social activity has to take precedence over a game, 'oh sorry I can't come out tonight/can't watch that film with you/whatever because my corporation is having a big mining op' is not something I could ever say.