why didnt the walkers get air support?

Ok, let me try my hand at an in-universe answer.

Manuverability.

Apart from takeoff, we have only seen X-Wings, Y-Wings, A-Wings and B-Wings in space. We have only seen Snow-speeders on planet. Maybe this has to do with manuverability. The speeders were obviously designed to operate within a planetaty atmosphere, which puts a different set of constraints on them. For example, the speeders appear to be quite nimble, circling around the AT-AT's legs with the tow-cable to trip them up. Perhaps the rest of the rebel fighters simply cannot manuver as well at those speeds.

I would think the same thing would apply to the Imperial forces. We've seen 4 forms of the TIE, Fighter, Bomber, Interceptor and Vader's. Never once though, have we seen them operate in a planetary atmosphere. My guess is that they would have the same constraints as an X-Wing. Very fast, but not manuverable at the slower speeds necessary for combat flying within an atmosphere, making them useless.

Now if you want to go EU, well that's different.

 

Very fast, but not manuverable at the slower speeds necessary for combat flying within an atmosphere, making them useless.

I like that answser. Atmosphere makes sense to me, though I'd like to hear an engineer's argument for or against.