Barack Obama ran on health care reform. It wasn't incidental to his election; it was absolutely essential. Not, to be sure, to the general election campaign, but to his nomination in the first place...Obama couldn't have run from healthcare reform, or tested the waters to see what's up - this was his platform. This was his foundation, what pushed him to his post. Ignoring that mandate would have alienated not only his base, but a big chunk of other liberal legislators.
In other words, attempting to pass health care reform was not a choice for Barack Obama. Any Democrat elected in 2008 would have done exactly the same thing. And given the similarity in the plans pushed by the leading candidates for the nomination, it's fairly safe to say that any Democrat elected in 2008 would have had a substantively fairly similar bill.
The bigger question in my mind than why Obama moved forward on such a difficult issue is how an issue that has such a liberal connotation in American politics was able to cause such a divide in a liberal party. Yes, I know the Democrats are a big tent and represent many ideas, but healthcare is a staple liberal idea, like supporting labor. Shouldn't there be some baseline from the party about it?