Those reasons being faction design.
There's a reason the Pershing is the way it is, what with a limit and no ability to decrew. The faction was designed with strength and weakness. The weakness being not having a Panther type vehicle. The strengths being high mobility, firepower, the ability to self heal armour and an ungodly supply of smoke for maneuvering.
USF are not supposed to outlast the enemy armour they are supposed to out maneuvering them. Okw is meant to outlast and they notably have the hardest time keeping their armour in repair (due to expensive engineers), not the easiest like USF (whom every tank IS an engineer)
Right, well "faction design" would have been relevant a few years ago but I doubt it's so anymore.
Simple reason is change, new balance/design focus and so on and so forth.
Most basic example would be the OKW, which were designed to have only 75% from what I remember of resource generation compared to other Armies with their mech HQ providing a conversion of fuel to ammo and vice versa, no stock MG, you could only call in a 34 from either the Luftwaffe or Fortifications doctrine, stock Sturmtiger instead of the Panzer IV Ausf J/H and you get the idea.
Also the USF originally wasn't intended to have a heavy tank from the looks of things and yet here is the Pershing, a late war US heavy tank that saw extremely limited combat in 1945 while the Sherman Jumbo was deployed sometime in 1944 with at all 250 of it's produced vehicles seeing action with various different armored units, the most famous of which would of course be the 2nd and 3rd Armored Divisions, "Cobra King" which had the first in Bastogne sign on it's side that relieved the 101st Airborne at the Battle of the Bulge being from this formation.
So again no, I don't agree with the excuse of faction design anymore, I just think it's shortsightedness and the fact that there is a huge missed opportunity here for a lot of new and interesting units to be introduced, stuff like the Wespe or Grille as self-propelled artillery units for the Axis being some others as well as the M4A4 Shermans for the UKF maybe, 17 pounder Achilles, Archer and probably a lot more.