Simple:
Selfishness is a selective trait that is mutually exclusive from social cooperation, which is also a selective trait.
The two hold each other in check with negative feedback. In an environment where almost everyone is highly social and cooperative, there are huge benefits to be reaped by selfish individuals, and they will propagate at the expense of their more cooperative counterparts.
This results in higher and higher proportions of selfish individuals until it reaches a point where there aren't enough cooperatively social individuals to take advantage of, and the advantage of selfishness is lost. Under these circumstances, individuals who are able to cooperate loyally end up with an advantage over all of the selfish "every-man-for-himself" crowd who lose out on the benefits of synergy and specialization. This reverses the momentum, moving the ratio in favor of socially cooperative individuals once more, until the high expectation of social trust creates too many opportunities for the selfish ones to thrive once more.
Rinse and repeat for 200 million years.
Thanks for the feedback. Makes sense to me. I think this is basically the Colman & Wilson model.