You're trying to draw a conclusion from a single piece of numerical data in a field that is vastly more complex than that.

In 2008 the other choice was McCain/Palin. The choice of Palin as VP nominee did a lot to cast McCain's judgment into doubt. Had he chosen a less preposterous person to put a heartbeat away from the launch/Rapture codes, he might have won.

In 2012 Obama had the advantage of incumbency and had shown himself to not be the leftist firebrand that the right had never stopped howling about, and Romney was just too unpalatably "retail" for too many voters.

Yes I have left his race out of the formulation because it had the same effect on both elections so it kinda cancels out.