drive over and over again in the same circles at high speeds... NASCAR is popular because a) doing the same thing 200 times is understandable and b) ZOMG the crashes!
NASCAR is popular because it's a form of motorsport that's easy to follow and it's closer to the common folk than IRL or Formula 1. Sure, you'd think that turning left for 3 hours is easy, but newsflash: It's not. Go ahead, give it a shot. I'm sure you can find a copy of Nascar Racing 2003 Season by Papyrus somewhere, or try it in iRacing. Then come back and tell me it's easy.
Sure, folks love it because of the crashes, but they also love their favorite drivers. And tell me that 42 cars racing 200mph 3-wide for a big sum of money and glory is not interesting.
I, for one, follow NASCAR occasionally. I'm not a devoted follower or a big fan, but I usually watch the superspeedway races and the road courses. Still, the most difficult ones are probably the standard 1-mile ovals. And Darlington. Ohh boy.