He said his reason was that his character wasn't developed as he'd been informed it would be.
5 months ago, the following was written about the role Mike Skinner had on The Grand Tour
In the Grand Tour, Mike Skinner’s role as “The American” play’s the part of the boasting, self-deceived race car driver telling bad jokes. The problem is, the Grand Tour forgot the most important element of developing a character; background.
The Grand Tour dropped the ball when it came to “The American” by failing to introduce us to Mike Skinner via a skit of some sort or another. If they had made a film where the boys had to compete in one of their famous silly races and were only able to finish it due to the heroic assistance of “The American”; we the audience, would have had the background we needed in order to relate to the role that their new staff race car driver has been asked to play.
and also, 5 months ago, it was pointed out that his role on the Grand Tour needed to be dropped to make the show better:
But sadly, Skinner is simply not entertaining. Whatsoever. As a bit of a NASCAR favourite in the States, he was clearly appointed to please American viewers, but at no point have we found ourselves even raising a smile during his appearances.
All that happens with each segment is Jeremy making some kind of gag relating to Skinner not liking communists and then cut to the driver moaning about how crap the car is. He genuinely looks pissed off that he's come to the UK.
They would be much better placed to employ the services of a charismatic and funny racer known the world over.