Matt Yglesias makes the partisan argument for DC statehood (but appears to be as puzzled as I am about why liberals didn't push for it in 2009); Andrew Sullivan makes the enfranchisement argument. James Joyner, and some commenters to my previous post, argue for giving the district back to Maryland, which solves the enfranchisement problem.
Yglesias refers to the principled argument...I'm not sure which argument he's referring to, but I'll toss one in. As regular readers know, I'm not a supporter of majoritarianism, so Madisonian elements of the American system such as separated institutions sharing powers, bicameralism, and various Senate rules don't bother me in principle (and indeed I think many of those features are excellent in theory and practice). However, there is simply no good democratic argument for the composition of the Senate, and unfortunately the two-per-state rule is also just about the hardest thing to reform. Constitutionally, it's as set in stone as anything. If everyone within the system agreed to work around it, one could devise a solution; for example, it's theoretically possible to move state borders around to make the states equal, or at least less egregiously unequal, in population. However, in practical political terms, that solution is probably even more implausible than a Constitutional fix.
Fortunately, the practical effects of Senate malapportionment have never been very important to partisan outcomes (although, alas, I don't have citations for studies of it). There are (and again, my apologies for not having the citations) however some policy biases in favor of rural areas. In other words, neither Democrats nor Republicans are helped, but rural interests are helped and urban interests hurt. And thus the case for DC statehood: it's a work-around to redress the overall undemocratic Constitutional bias in favor of rural areas.
Beyond that, I think the case against statehood is weak. Yes, the District has a tiny population, but it's not tiny compared to Wyoming or North Dakota. Sullivan links to questions about fiscal viability, but I do suspect those concerns could be dealt with (given a Congressional majority large enough to support statehood in the first place). Two commenters brought up the 23rd Amendment as an issue, but I really don't think it's a serious problem: if and when Congress approved statehood, I suspect everyone concerned could rapidly pass a new amendment repealing the District's Electoral College votes. Who would (at that point) oppose it?
None of which, however, answers my original question, which is why liberals and partisan Democrats didn't push statehood. For that, I need to hear from liberals and/or partisan Democrats (not Sullivan, then) who didn't support it (not Yglesias, who I think has posted quite a few times about it). Anyone?