The reason you see almost all 21.5/lipo cars is that everyone wants to have a lipo pack they can charge and forget about. It is also easier to be fast with the combo since it has a lot more torque. If your car plows around the track it will still be ok fast. It does take more time to get the car right to work well with the 4 cell combos. You cannot run junk packs and compete with the lipo cars heads up.
This summer I raced my lipo/21.5 car at an outdoor race where I was in a 27t/4cell sandwich for the top 3. Both of the guys who drove the other cars are excellent drivers, and had good stuff to power their cars. I just charged batteries and fooled with my suspension
They had to worry about their motor/battery stuff in addition to the car. That is why you see so many lipo cars, there is less to do. It is also easier to be fast.
Can you guess which cars had what motors?