I would say because its not really fair to Ostheer. It punishs T3 far too hard. It really does. Fighting a Jackson with T3 is akin to trying to kill a King Tiger with Shermans. Sure you may get it EVENTUALLY but youll suffer heavy losses in the meantime.
Making it weaker against Mediums and stronger against heavies solves two problems. You always have the option to buy one if he goes Tiger but if Ost goes T3 then Shermans will do (which are much better tanks then the Jackson imho anyway)
It gives Ost more viable options. While solving a problem for USF at the same time.
This is why I wouldn't mind for a price increase and penetration increase, I just personally think nerfing Jackson's damage will make USF AT (already limited) weak against heavies, especially against OKW. If we need to address Osth vs USF than this is a different issue and requires an over haul for every single faction as USF faces problems vs OKW.