Probably because in GT Sport, you could create a custom 24 hour race with the x-bow on a certain Tokyo track against gr. 4 cars, put a rubber band on the accelerator button, and win every time, making like 5 million credits per day while AFK.
The performance grading system on that game was based PURELY on horsepower, so it didn’t take into account how light the x-bow was, and their dumb physics model let you wall-ride around the entire course at full throttle.
Rather than really fixing the physics or addressing the exploit, they nerfed the everliving heck out of custom race payouts, making them completely worthless for every single player at all times.
It’s the laziest possible solution that punishes everyone.