Adam Serwer writes that "if Democrats can't repeal a policy more than two thirds of the American people, including a majority of conservatives want gone then they can't expect people to vote for them":
In December the Defense Department is reportedly set to release a study showing that, like the American people, most servicemembers aren't opposed to gays and lesbians openly serving. That's in contrast to the vast opposition of most servicemembers to racial integration in the 1940s; if Truman had insisted on staying his hand until a political climate as favorable as this one had come along, integrating the military might not have happened until decades later.
Truman ended segregation in the military because it was the right thing to do, despite the fact that it was unpopular. Ending DADT happens to be both popular and the right thing to do, and Democrats today still can't get it done.