It's quite easy to figure out. 43 cars running at the limits of an oval, whether rain or no rain, is much dangerous crashing into the wall than road racers running at the limit whether rain or no rain. Oval racing is done at much higher speeds and there is no run-off areas, no sand, no tire barriers, etc. There is the softwall on track corner, but it is entirely different scooting head first into a softwall with concrete behind it at 200 mph, than it is running into a tire barrier after being slowed down from the sand at less than triple digit speeds.
It's all about safety and nothing else. It simply isn't safe to run oval speeds in the rain. You spin out on a road course, and you do just that, spin out and stop. Spin out in a top level stockcar oval course, and you are heading into a concrete wall at whatever speed you were doing.
[OT]The trolling of some members in this community in any topic regarding NASCAR, stockcar racing, or oval racing is getting quite a bit old......