Because the content of what the debate is doesn't really matter that much. I mean, look at Romney. He really didn't say much of anything, yet he won last night.
Also, they could be thinking that it doesn't matter who won the first leg, it only matters who won the last leg. If Obama dominates the last two debates, everyone will forget that Romney won this one.
Basically, it boils down to "What have you done for me lately?"
Sure, the absolutely most optimal narrative isc dominating all the debates, or even two of the three, and Romney's in pole position for that. But don't underestimate the value of winning the last debate. That's the one that will have the lasting impression, because it's the last one.