Scott Walker is out. He joins Rick Perry on the list of Republicans who officially announced their presidential candidacies and later withdrew. Obviously, we still have quite a few candidates left in the race, but this was a big one.

I'll admit that I expected Walker to be a stronger contender for the GOP nomination. On paper, he looked great — a governor elected multiple times in a blue state, even surviving a recall, with a record of conservative achievements in office. But on the trail, he was a lackluster candidate, and though he won some early key endorsements, those stopped coming in, as did the money.

I will still claim, however, that at least as of last week, Walker was a more likely choice for the GOP nomination than Donald Trump, Ben Carson, or Carly Fiorina. So was Rick Perry shortly before he dropped out. I'll admit that's hard to prove. But if it's true, why did Walker and Perry drop out before those others? Why would stronger candidates drop out earlier than weaker ones?

Precisely because Walker and Perry are serious politicians. This is a career for them. Walker, in particular, still has several years left in his term (in an office that isn't term-limited), and he might make a run for U.S. Senate some day. He might also think seriously about a presidential run further down the road.

He probably could have strung out his presidential campaign a few more months on a shoestring budget, and maybe even found a few eccentric donors to back such an effort. But he's a smart enough politician to see that probably wouldn't have succeeded, and he'd have been humiliated in the early primaries and caucuses and just angered some donors who would have seen him as a waste of money. Better to show some discretion than go all in on a suicide mission, especially when he's only in his mid-40s.

There's also the important matter that experienced politicians can read the writing on the wall better than inexperienced ones can. An inexperienced politician can easily be snowed by a consultant who tells him that the silent majority are behind him even if they're not showing up in the polls just yet. An experienced politician can interpret cues, like which activist groups and elected officials are supporting him and which aren't.

Compare that to some of the other candidates, who really have nothing to lose by staying in the race. These are not serious politicians and they have no political bridges to burn. Trump may have damaged his brand a bit with his nativist outbursts, but he still obviously knows how to draw a crowd. Carson and Fiorina will not harm their political futures by losing in primaries. Even Ted Cruz can stay in for a while; it's not like he's about to get anything major accomplished in the U.S. Senate. He's already burned what bridges he had there; his whole career is about making bombastic claims from large daises.

I continue to think that candidates like Chris Christie, Rand Paul, and John Kasich are long shots for the nomination, but precisely because they're politically experienced and have things to lose, they're more likely to drop out early. Most of the other candidates (with the notable exceptions of Jeb Bush and Marco Rubio, who have legitimate shots at the nomination) may stay in the race long after it's obvious to everyone else that they've lost. They'll stay in for precisely the same reason Zack Mayo stayed at Navy flight school: they've got nowhere else to go.