Why, exactly, did he decide to leave?

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And he won't get a career if he's devoting the majority of his year to filming and promoting Doctor Who.

Hes moving to L.A, so he wants a hollywood film career.

Humankind cannot bear very much reality. ~T.S. Eliot

He is young and does not want to be typecast.

Smith has a raw character face. Surely a little hard to cast, but nevertheless something than can be done.

I think the characters that be wanted someone older for the 50th celebration and Smith told multiple times that the shooting schedule with 10+ month a year is quite harrowing, especially since Who is a show with a wide range of filming locations you have to travel to.

He still has to promote Who this year, then its up to the stage.
http://www.dailymail.co.uk/tvshowbiz/article-2447892/From-Doctor-Who-Psycho-Matt-Smith-begins-rehearsals-musical-transports-Eighties.html

I guess he really has to move to LA before he even can begin to look for something new.