You don't have to "pass" a background check to be president. Doesn't mean background checks weren't conducted at various times throughout Obama's political career. And you can be sure that any information that would have compromised the democrats' chance of winning the election would have been passed along. Obama's associations were well known on election day and 69 million Americans voted for him anyways.
But, as I posted above, I don't expect birthers to accept lucid, rational explanations. For birthers, the fact that no damning material was released on Obama during his candidacy is just part of greater conspiracies and cover-ups.