iRacing staff have also been rather vocal in the past about how much of a pain P&M were to work with when getting the Corvette C6R into the game, and how much of a pain they were to deal with when considering getting later Corvettes into the game. It was a bit of a surprise and a miracle to a lot of people when it was revealed the C8R would be coming in 2020, which is still over a half decade since the C6R debuted in the service.
Look up any info (before 2020) about new Corvette models coming to iRacing, and it all says the same thing.
It's absolutely no secret they're trouble to deal with, and considering Gran Turismo has to contend with including literally the entirety of car culture when adding new cars, I can hardly blame them for deciding not to prioritise something that's notoriously a hassle to deal with, and that's assuming P&M haven't just blacklisted them for some arbitrary reason (as far as I'm aware, the whole reason P&M were such a hassle with iRacing is because they refused to give out the intricate details the car needed for a proper representation).