Yes, realistically we know why. Sarah Palin isn't qualified. Joe Biden certainly could do this as a stand in for Barack Obama.
So, I don't really know what John McCain's motive is for suspending the campaign. Perhaps he's willing to lose the election so that this bill can pass. Fine with me. The fact that there is no back-up, that he can't manage both at the same time (even at some lesser level for the campaign), tells me he shouldn't be president.
I suspect this will be the defining moment of the election. What happens in the next week as a result of the suspension will set the remaining course of his candidacy.
If he can be seen as the crucial negotiator/voter in the bailout, he wins. If he doesn't, Obama will eat his lunch. I don't think Reid and Pelosi will let him play that role. (It might be low, but it's politics.)