because
Stating the obvious I'm sure but Vmcg = Minimum Control speed on ground, put simply, below this speed, in the event of an engine failure, you will be unable to control the aircrafts direction of travel.
So ., putting a couple of numbers on this, for the sake of example say Vmcg is 150 knots but V1 is 130. If you have an engine failure at 140 knots you can't stop ( 'cos your above V1) but you cannot control the aircraft either.....
edited to add that lowandfast beat me to it