It was actually a bit limited by the gearing/transmission. The car can go faster...
Granted, what that essentially means is that the engineers went "balls too big, let's cap this in case the tires explode or the car flies off the track."
Seriously though, the driver is putting it all on the line here. Driving a brand new car balls-to-the-wall at 300mph around a curvy race trace ... it's basically like being a test pilot with no parachute or margin for error.
I'm guessing the weight of the drivers balls is limiting factor here. Any lighter and the car would not be able to go so fast.