[Si Coe]

The thing is that something like an Ezrun brushless system can be bought for very little money, and at a lot of tracks is more than enough to propel a good enough driver into the A. Brushless offers so much power even racers on a tight budget can have a car as quick as anyone else.
In contrast, a restricted brushed class would soon develop issues with people having superior motors or esc's - especially when you are talking RTR's were there is a huge variation in the quality and wind of the supplied motor.

Nobody says you can't use brushed in racing - some people still do and in some applications people swear its better, but cheaper and simpler it isn't.