There are a number of reasons the cars are restrictred...the first and foremeost is safety. 14 years ago (1988) the last unrestricted race was run and then the fastest cars were reaching 212 mph around the track. At that speed, if for some reason they get turned around, they lift right off the track and just keep going sue the the aerodynamics of the car. To help prevent this, they have roof flaps and hood flaps to break up the airflow over the surfaces of the vehicle and keep it on the ground. However, this is not a perfect system as seen with the crash Ryan Newman was involved with Sunday.
Now I know Indy and F1 cars run well over 220 mph, and they stick to the ground. However, those cars have a toally different set of aerodynamics...and most importantly the "hole" they punch in the air isn't near as large as the one the winston cup cars punch. Also...there is no "stock" car in the world like those cars, and they are designed with the high speeds in mind. Conversly....your chevy Mote Carlo at the local dealership isn't designed with the 200mph speed in mind. The cars NASCAR races are supposed to be "stock"...or have body styles identical to that which you see at the local dealership.
That's the nutshell answer...there's more to it than that, but this is getting to be lengthy
O.K...anyone else have some questions