It seems a pretty strange decision to me, not that I know a great deal of the politics of English cricket.
He is pretty old- all of 32 (that’s tongue in cheek- these days guys start getting looked at as over the hill when they are middle 30’s). And really, he has not played a huge amount of International cricket- he was a late starter.
Any theories about it? He has had a pretty adventurous career. If he was thinking of making big bucks in the IPL I thought he would have quit Test cricket first. (Not that he won’t make big bucks there anyway- maybe he is giving himself more time to play in India).