If NASCAR had, on average, the same amount of passing as an F1 race, I'd say go for it. There would be little to loose. F1 this is not, and the quality of the racing, for the drivers and fans wouldn't be there on a wet oval.
There is no definative way to say, this is a safe amount of rain to race in and this is not, so NASCAR avoids the issue. The ACLO and FIA feel free to flip to flip flop, as seen by this years Le Mans and Nurburgring F1 race.
Pit Crews. The nature of NASCAR is that pit stops take place in large packs. Also, Pit Crews stay on the pit wall until their car is in its stall, when they then leap onto the road. Sure, you could slow the pit lane speed limit, but 30-plus 3400lb cars (240pit crewmen) + standing water + skinny wet tires + narrow pit lanes = unnecessarily dangerous.