In many respects, the times and the man just didn't jibe. Johnson is extremely well-liked and well-respected, and is a Washington insider, and a man very much identified with mortgages. And Obama is an outside-Washington change guy. The veep choice is extremely important.
Something changed; yesterday, this was a "game" and irrelevant; today, it was important enough to accept Johnson's resignation. Was he pushed? Did he resign voluntarily?
Will the team of lawyers that Johnson uses to vet the candidates -- the usual suspects who've done this for cycle after cycle -- stay on board? If not, what's the cost of finding a new team of trusted folks?
Republicans have their head. Payback time for all those folks who were forced to leave their positions on the McCain campaign. This war of attrition saddens folks in both camps.