I guess it's a mixture of several things.
His wife is said to have put a lot of pressure on him to move to an english speaking country, or at least a fashion capital such as paris or milano.
Last year he has opened a football academy in LA, and probably wants to be closer to it.
Money should have something to do with it. You don't always turn down a 250 million $ offer...
About his not going to a good league, you might want to take into account that he isn't the best player around (read another answer of mine on yedda, right here). A player at his age and level can't get a new long term contract in a really great club, that can pay him what he's used to. So his options are moving to a middle table club in either spain, italy or england, retiring at a Qatari or Emirates rich club like some other footballers do, doing the same in the US, trying to promote football using his being a celebrity, or retiring now.
I'd say he made quite a good choice...
