Not enough time

That's the answer to the question below.

"Why haven't other rodents evolved the ability to aviate?"

Just not enough time. Look at the time frames for the evolution of birds. Took a hell of a long time, a long, long time ago. Mammals are much more recent. They just haven't had time. Our (Homo Sapien's) ability to observe such a change is pitifully short.

Here's my thesis: more mammals will develop flying abilities. Let's leave the planet - everyone. OK, come back in, let's say 100 million years. My bet is that you'll find plenty of flying mammals. Lot's of little annoying ones. And a few, big, scary ones.

First, flying squirrels develop from gliding to true flight. If they're successful, they'd multiply greatly. Next, you'd have a predator that would take to the air after them, let's say a fox variety. Other, fast breeding species like rabbits might find it useful to get off the ground, which would bring bigger predators along with them, say bobcats or coyotes. But the birds won't give up their dominion very easily. There are a bunch of very effective preditor birds that would just love to find the air full of flying furballs.

But you know what might be more likely? If humanity abandoned the planet for that long, I'm betting another hominid species might just step up and take our place.

On second thought, at the end of the 100 million years, let's send Charlton Heston back first to check and see how things are going.