Pine has been on a roll ever since starring in last year's Star Trek reboot courtesy of JJ Abrams. He's already been tapped to take on the role of Jack Ryan in Tom Clancy's famed espionage series of films.
The reason I think Pine might be the choice is simple: fanboy cred. Ignoring the fact that he's got nearly two years until his big projects come back around, the guy has gained a serious following after Star Trek's surprise success. He's got the clean cut look, the right build(remember Steve Rogers was a lanky kid initially, not a bulked up musclehead), and the screen charisma for the role. While I would prefer Human Target's Mark Valley to play the part, this is a good second choice.