I think it's a case that he wanted to establish Villa as a premier league force. I mean he has improved them quite a lot in his time there but I bet he feels like it's just not possible to compete with the likes of the Big 4, Spurs and esp City. I think the fact he is still losing his best players to rivals (Barry, soon to be Milner, who's next? Young? Agbonlahor?) was a big reason for it, also SSN say that the recession has hit Lerner quite hard so it could also be to do with the lack of reinvestment in the team.
My take on it anyway.