It's for safety reasons. The new Nascars are basically roll cages on wheels. The old ones were pretty much STOCK cars, hense the name STOCK CAR RACING. But in the late 60s cars like the Dodge Charger Daytona were breaking the 200mph mark in almost street trim, but there were too many accidents, and too many people getting hurt or dying. The safety rate today is much better.

Also back in the 60s cars were actually fast, 400hp cars from the factory were a dime a dozen. Now today, really tell me who would want to watch Ford Taruses and Dodge Intrepids driving in an oval, they probably max out at 120mph, wow real exciting.