I’d rather have a 180 lb kart land on top of me in a crash, than a 210 lb kart. So it’s partially a safety thing. Karting is supposed to be lightweight and nimble and since it’s inception, the karts have slowly gotten more bloated.
Karting was never meant to be a sport for big guys.
Also, I’ve been trying to gain weight for my whole life, and I’ve been lucky (unlucky) enough to have a killer metabolism so I can’t really do much about it. Plus, some of us are at the point where there simply isn’t any place to put more lead.