Because these are different games, they have different purposes. The PE Scout Car was supposed to be the "jeep" vehicle (no fuel cost, good vs snipers) that could also go out and capture territory or lock it down like an OP, and the Armoured Car was a different unit with a meant for light combat and support.
In CoH2, it's one unit instead of 2, and it's role is more combat oriented even without the upgrade. The problem is, that role is nullified partially by AT Grenades and nullified completely by Guard Rifles.
Well it's also that the Scout Car costs fuel. I get that the M3 also costs fuel, however the M3 is T1 and can carry units around. Also, Soviets can (and often do) tech to T3 without building T1. Germans will almost always build both T1 and T2, so you have to tech twice and build two structures before you're even thinking about armor. Which is just to say that, unless the Soviet player is going for a REALLY fast T70, the fuel cost of the Scout Car is not really commensurable with the fuel cost of the M3.
It doesn't bother me that Guard Rifles can nullify the SC, they are dedicated light AT, of course they should be able to nullify the SC. The problem is that you don't even need AT nades as things currently stand, in a prolonged engagement from behind light cover a vanilla conscript squad WILL take out the scout car (whether it has been upgunned or not). That's simply unacceptable. Either remove the fuel cost and keep them as is, or bump up the MP cost and make conscripts pay for sticking around without AT nades.