Newborns have some ability to roll over that is lost and then regained, so it's not that they are physically incapable of rolling over.

I think the problem is they lose a lot of weight early on (can roll over) and then begin to fatten up quickly. The calorie intake relative to body size is immense and for a few months a baby just doesn’t have the muscle strength to support its newfound fat composition (loss of ability to roll) in addition to their heads growing rapidly early on and not having the neck muscles developed yet.