The reason for this is USF has only ONE AT vehicle in their core line-up unlike all the other factions.
-Ostheer has Panthers and Stug Gs
-Soviets have an SU-76 and SU-85
-OKW has Panthers and Jagdpanzers
-Brits lack dual AT vehicles (AEC doesn't count) but compensate by having Comets or Churchills.
Furthermore, almost all Allied TDs outsight the Jackson in some way through abilities or upgrades. Fireflies get 45 with the TC, SU-85 has focus sight. Jackson only slightly better sight at 40.
USF don't have alternatives.
USF can easily deal with mediums with Bazookas, Stuarts, ATG and Shermans. They do not need a units that makes German mediums obsolete.
Keep in mind that Stugs/Su-85/Su-76 can be flanked by medium while the M36 has bonus vision and a turret making it allot harder to be flanked.
Designing a M36 that can deal with all vehicles is simply bad design, especially since USF infantry can deal with most axis infantry.
M36 should either be designed as "heavy tank counter" or it should simply swap places with the doctrinal m10.