There's a theory making the rounds now that when the meteor hit it only killed off the big stuff. That there were plenty of smaller reptiles running around, about the same size as all the mammals at the time. The reptiles should have regained their position eventually, but they didn't, because the mammals could feel love. They could be utterly, even irrationally devoted to their mates and their offspring. It made them more likely to survive. The lizards couldn't do that. The meteor hit gave the mammals their shot, but it was love that turned the tide.