Kam, there are several reasons, but the main one is that NASCAR runs almost exclusively on ovals. The cars are not equiped to run in the rain, and though they do have contingency plans for the cars to run in the rain at the road courses, they still prefer not to. Tires are the real reason. F1 races have fewer cars running and the teams can pick and choose the brand of tires they run and all of those manufacturers make specific rain tires for those teams. NASCAR at the present, only allows one brand of tire, Goodyear and it's hard enough for Goodyear to keep up with the various tires the series uses and if they were to then add a rain tire for the various tracks etc, well the cost would be out of this world.