To me, the only real explanation for Democratic behavior is this. The party's leadership and political thinkers simply can't conceive of national security issues as anything other than a source of potential political problems to be coped with, never as a set of potential political opportunities. Since congress can't unilaterally end the war, then, there's no reason to have a confrontation with Bush; national security debates are just pure downside. Overwhelming polling data backing the liberal position isn't a reason to go on offense, it's a reason to think Democrats can succeed in slinking away.
I know the conventional wisdom on the left is that all the Republican politicians who are talking about September as a hard deadline for the surge to show results will end up falling in line behind Bush when the crucial moment arrives. But if the GOP is still staring at numbers like these come the fall, well, I don't are how many Victory Caucuses Hugh Hewitt founds, a lot more Republicans besides Chuck Hagel and Gordon Smith are going to decide that Iraq isn't worth their jobs, and as Rod says, the end-the-war wind will be at the Democrats' back from then on out.
From my "give the surge a chance" point of view, waiting till the fall is the right thing to do, and from the country's perspective there's a lot to be said for having a controversial occupation come to an end under something approaching bipartisan auspices. From a purely partisan-Democratic point of view, though, I take Matt's point: If the Democratic Party were conditioned to think of foreign-policy debates as things to be won, rather than avoided, I don't think you would have seen so swift a climb-down on a question where public opinion is clearly on their side, and I certainly don't think you would have seen so much fretting in the Democratic ranks about the need to get a bill to Bush's desk by Memorial Day Weekend. E.J. Dionne is right that ultimately, the Dems can't end the war without defunding the troops, and they aren't going to do that (yet) - but he's also right that as a matter of tactics, the Democrats had more to gain than to lose by forcing an unpopular President to veto popular legislation at least one more time. It's hard not to think that if the Republicans had a wartime issue where 63 percent of the country agreed with them - which is the percentage of the public that wants a timetable for leaving Iraq - they would be thinking more about how to go for the jugular, and less about the risks associated with having a President whose approval rating is mired in the low 30s accuse them of being unpatriotic over a holiday weekend.