1) As Congressman he can devote himself entirely to the crucial work of taming the federal budget. A president can't be so single-minded.
2) He'd be a far-from-flawless candidate. As Ramesh Ponnuru wrote,
Right now, conservatives think of Congressman Ryan as a bold, free-market visionary. Within weeks of his entering the race, he would be redefined as the longtime Washington fixture who voted for TARP, the prescription-drug benefit, the auto bailout and other bills hated by Republican primary voters.3) We don't need another commander-in-chief for whom national security is of less than primary importance. Here's the headline of a story from the NYT's front page on 9/11:
KEY LEADERS TALK OF POSSIBLE DEALS TO REVIVE ECONOMYUrgent stuff at the time, and by the afternoon no one was thinking about it; the world had changed. Ryan's natural focus, like Obama's, appears to be overwhelmingly on domestic policy. That's not the kind of president we can afford anymore.
Ryan seems a very good man, and I'll vote for him if he's nominated, but we're better off if he stays where he is.