Partly because he's not all that far from the middle to begin with, despite all the insane rantings about him being a marxist/communist.
In the past he had an excellent reputation as a negotiator/mediator.
As President of the Harvard Law review he was reknown for being able to bring opposing sides together.
I think he genuinely believed that in the midst of the multiple crises he inherited there was a very real possibility of increased bipartisanship. I mean logically thinking who in their right mind would put power politics over the well being of a nation in crisis? As he came to find out, many in the GOP would.
He also has to deal with a force that's even more powerful than the presidency, or congress: the money lobby is going to fight any major changes to their protected financial system, so any financial reform is going to be forced to steer a very middling, milquetoast course - no matter the risk of another similar meltdown.
He is also a student of Lincoln, and Lincoln, at least at the outset of his presidency, had the same goal of bringing disparate factions together in a time of crisis. Lincoln even named a couple of very powerful men who opposed him politically to his cabinet, which as I recall was all but unheard of up until that time. Obama also named opposition party members to his cabinet.