Very good question. Maybe the ecology of the early whales and/or the
persistence of the dyrosaurid crocodiles and/or the lower productivity of
hot seas are to blame... though even then, the pinnipeds didn't appear
immediately at the beginning of the Oligocene.
I'm confused though, as to why bats didn't face the same challenge
when invading the bird niche. (Is it because they're nocturnal?)