>>64149682
>blown up by X-Wing blasts
They get two outs on that one. One is that we never actually see how well aimed starfighter fire affects them in ESB. That's pretty weak though, since if it works you'd assume they'd just deploy X-Wings instead of messing around with snowspeeders and tow cables. Personally I'm okay with the idea that they're vulnerable, particularly if you go for the neck, but it should take at least a couple passes.

The better out is that in Rogue One what we see aren't actually AT-ATs. They're a utility variant, with longer legs and a big internal cavity. If they're for behind-the-line use it's totally reasonable that they wouldn't be as well protected as the frontline vehicle. Enough armor to withstand small arms fire in case of an ambush by irregular forces but not enough protection to withstand heavy contact is pretty consistent with real life utility vehicles.
We also see sustained fire from the door gun of a U-wing being enough to take out the knee joint on one, which I'd take as supporting the idea that they're just overall not as well protected as a conventional AT-AT, more than that they nerfed the original vehicle.