Very simple: because the racing would be awful.
The reasons for not using rain tires at ovals are obvious: if the cars didn't slide right up the hill and into the fence in Turn 1, the treads would be sheared off within a few laps. But how about road courses, which don't have similarly severe banking? Why couldn't rain tires be used there?
Well, for starters, it's not just about the tires. If you're racing in rain, your visibility through your windshield is going to be terrible, which means slower speeds and less competitive racing. There's also the issue of consistency; there's a huge difference in grip and feel for the drivers between a driving rain and a gentle spring drizzle.
That uncertainty, really, is the main issue, as NASCAR noted the last time Watkins Glen had to postpone a race to Monday ... two years ago. "For the level of competition that we have in the Sprint Cup Series and as the stakes continue to rise from a competition standpoint, a sponsorship standpoint, a championship standpoint, we'd be best served to run the Sprint Cup Series on dry race tracks," NASCAR spokesman Kerry Tharp said at the time. "We feel like that would be better for competition, that would better for the level of racing that the fans are accustomed to seeing."
"We have enough challenges trying to stay on the track when it's dry and I can't imagine what it would be like in a Cup race if it was wet," Jeff Gordon said at the time, adding, "One of the biggest reasons, I think it would be fun to actually drive the cars in the rain if you get a consistent rain and you can feel the grip level, but as you saw [in a Montreal Nationwide race run briefly in the rain], the windshield wipers don't work, the de-fog doesn't work."
Gordon addressed this issue again Sunday in a tweet: "To weigh in on rain tires. It wud b exciting as driver but we'd run more laps under yello then green. I drove once here in wet. #handfull"
So there you have it. Rain racing: nice in theory, problematic in practice. Like it or not, NASCAR is looking to get the best racing possible at every track, even if that means waiting until Monday or Tuesday. Your thoughts?