Why? He would have won handily. This would have set him up perfectly for 2012, or for 2016. Why'd he do it? He's been an excellent governor so far.
Because bad things are coming. Governor's are in charge, they get blamed, they must act. Senators just vote. It's a choice anyone with higher ambitions would make.
Such as? To FL, specifically I mean. This move is dumb, IMO. Governors are Kings of their state and he's King of a huge one. So what bad FL stuff?
But as someone noted, maybe he's got more to choose from in DC to fulfill his desires.