Because KPTV vehicles cost something like 1/12th the cost of tanks they were killing and were spammed, Stuarts dont have that, they cost at most what, 1/3rd the cost of the german vehicles and aint that spammable, at least not without completely destroying your own ability in phase C.
Again, becaust the T34 was spammed and abused by people going at a serious speed down roads, you dont have this in steel division.
I wouldnt call the stuarts super cheap, they dont cost 1/10-1/18th the price of the panthers in steel division, kind of a moot example simply from that alone. The problem in wargame was spammage, you simply dont get that in steel division, this is a different scenario entirely with significantly different unit costs, counters and game mechanics.
If you could get 18 stuarts for the price of a panther you might have a point, but you dont.
But it can pen it in-game, making it a you are not playing right if you let it happen lol..... You need to remember "it is a game, not real life".... As far as i remember, didnt they say its authentic, not realism with a dash of gameplay?