Money - I only work part time at the moment and have bought 13 games with my own money this year (13 360 games and 1 DS game) and only two of those were brand new (Fifa 09 for £30 and GTA: Chinatown Wars for £23) 2 for £30 offers ftw!
Then again if I DID have money then that'd mean working full time which would cut down my gaming time at the moment by quite a lot!