The SR+ battery has PLENTY of power to propel an AWD car. This just simply isn’t a factor - though you’re right it may reduce the real world range to the point of being unacceptable.
It’s purely economics. It’s no secret Tesla has been cell constrained for the Mode 3 for quite some time. Thus, they’re valuable, and if you want a long range car and all the extra cells it comes with right now you can only get it in the premium AWD (ie higher margin) version.