the reason is because, he does not want the hassle of being famous he wants to continue a normal life. this was the same case for the man who did the voice over on "live and kicking" on cbbc a while ago his name was mitch.
its becasue Clarkson is an arse; him and Tiff Needel fell out becasue Tiff got to be as famous as him (and can drive rings round him to boot), so he didn't want that to happen again and insisted on keeping yje "stig" person anonymous.
I think its such a crock, I'd much rather listen to someone who knows what he's talking about than bonehead Clarkson. The program would be great without him in it.