As organic_code pointed out, blinding the drivers is a real possibility. Also, races are fairly long and holding a mirror up for two or three hours doesn't sound like a lot of fun. New drivers in the sport have no chance of winning (see last years Daytona 500). Never mind the problem track owners will have selling concessions to a group of people who cannot eat / drink for most of the race.

I want to watch a driver's skill, not the energy output of his / her fans.