the problem is you want the weight to be after the suspension, not before, so that the unsuspended weight is minimal. Putting the motor inside the wheel make that not possible, and motors are quite heavy, even electrical ones.
I believe that makes sense since there is a lot of alternating upward and downward acceleration on the wheels while following irregularities of the road. I guess the resulting constant change of momentum would be bad for stability of the suspended part, and would wear out the suspension faster.
