Because General Veers is a commander but General Weiss is just an upgrade. At first I thought maybe it's like Wedge, but Wedge is a fighter pilot and any rank he holds would apply to fighter combat not ground combat. Weiss, he is a ground combat General, there are numerous examples of generals leading from tanks or vehicles, in star wars we even have Veers in Blizzard 1 (the size of the AT-AT precluding him from leading from it.... for now.)
So, would any of you want to see Weiss given a commander model, to be stuck in an AT-STs hatch and made a legion commander?