There are many reasons, but to make it simpel: V1 is the speed, when an aircraft (mostly airliners) accelerate down the rwy for take off, when the aircraft now have so much speed, that it would not be able to stop on the remaining rwy, if a serious mailfuntion occour. So before V1 you stop, after V1 you continnue the takeoff roll. Vr is the speed that the pilot rotate the aircraft, to get it airborne. If Vr was to be lower than V1, it would be possible to get the aircraft airborne, have a mailfontion, then land and stop on the remaining runway. I can not think of any rwy in the world there are that long.
The limiting factor are that the aircrafts needs a certan speed to get airborne, and the runways can only be a certan length. There are many other factors such as rwy surface, density, system u/s on the aircraft etc.etc.