You can blame that on open primaries. Had the primaries been closed to party only, McCain would have been an "also ran" in the primaries, instead of the general.
Open primaries were a contributing cause toward McCain's selection, I agree.
Yet, knowing the rules ahead of time, neither Romney nor Huckabee nor Giuliani nor Thompson was inventive enough to arrive at a winning strategy.
No need making excuses for any of them. If they couldn't figure out a way to beat McCain, they're not going to figure out a way to beat the Democrat. Thus, they're not worth putting forward again.