I think that some of them has retired or got fired by the sponsor. Here is the stock cars that appeared on the LA500: Rev-N-Go Racer Dino Draftsky Rev Roadages Ponchy Wipeout Murray Clutchburn Darren Leadfoot Ralph Carlow Reb Meeker Brian Spark
Here it is what happened with all of these 9 stock cars:
After a very hard race, Rev-N-Go Racer was the best stock car racer in the race. After finishing the race in 11th place, the team decided to buy a simulator fore him. After he fails miserably in the new simulator, the team decides to fire Rev-N-Go Racer.