More than anything, I think it has to do with the venue. McCain is great at townhall meetings and Q&A's. He's also a decent debater. But he is not a good speech-giver.
This is problematic because McCain's strength has always been his authenticity. But when he delivers a major speech (as opposed to off-the-cuff remarks), he lacks the same level of believability and passion.
The good news is that his debate performances will probably have a much larger impact on whether or not he wins in November than his ability to give speeches.