Nate Silver was ahead of his time on those advanced metrics but he was undoubtedly one of the pioneers. If people had been more receptive to his brand of statistics five years ago he likely never even goes into politics. Sports was his first love (along with poker, which he goes out of his way to not mention these days, unfortunately. He even obscured his username on the largest poker forum[2], so it would be harder to find with search engines).
Completely random thought that probably means nothing, but is worth noting: he's openly gay[3]. With the tide on gay athletes turning sharply and quickly, I have to think that factored into his decision. Perhaps he is hopeful he can play a role in that, being the champion of the gay athlete or at least the one guy at ESPN they know will be on their side.
[1]http://en.wikipedia.org/wiki/Wins_Above_Replacement
[2]http://forumserver.twoplustwo.com/members/4548/
[3]http://gawker.com/5969477/sexually-gay-but-ethnically-straig...