NASCAR has a lot more money than me, so they can tell me to forget it.
A successful NASCAR driver brings a sponsor with him. The deal is he gets the sponsors name on TV.
It has been observed here on CT, more than once, that the slower and more southern you talk the dumber you are perceived by yankees. Because of that, there is a lot of pressure on NASCAR racers to sound like "everyone else". I hate it, but that is the way it is. Jeff Burton could have been raised in Los Angeles for all you can tell from his voice. I'll bet thousands of $ of "speech therapy" are the reason.
Indy Racing has Brazilians, Brits, South Africans, Australians and they all sound about the same.
If you are depending on sponsors and TV for your money, you better sound neutral.