The increased fuel efficiency would crush the competition. Audio raced hybrid diesels at Le Mans and finished 1-2. Then they changed the rules so fuel efficient cars would be at a disadvantage.
PS "stock car" means a car that is manufactured for sale to the public and not strictly racing. Hybrids are stock cars.
The problem with this is that hybrids are still a new, fresh idea. Most of the public still runs on gasoline. See, NASCAR isn't going to switch over when it's still a controversial topic. Only when hybrid cars are the majority on the streets will NASCAR hop on the bandwagon.
Nascars are supposed to be made after stock cars, hybrids are not stock cars, sadly they are the cars of the future, they should stay petrol as long as we race them, no race car should be hybrid there is no reason for them to become one now. So why would they want to change, that would actually do no good because Nascars are rarely ever still long enough for the start/stop technology to work.