The purpose of the car's suspension - the springs and shocks - is to decouple mass from the wheels. This allows the wheels to travel up and down over bumps and dips in the road surface quickly without large forces being exerted, which in turn makes for a smooth ride and good traction between tire and pavement. The more weight you move from the 'sprung' part of the vehicle - everything that's suspended by the springs - to the 'unspring' part - directly connected to the wheels - the more this is compromised. You can think of the extreme case where you take all the vehicle's weight and move it to the wheels: then it's like you don't have suspension at all and you're bumping and jarring across every imperfection in the pavement. Moving a lesser amount of mass to the wheels is basically a lesser version of that. (Likewise anything you can do to reduce unsprung weight - lighter brakes, wheels, tires, etc. - tends to have an outsized effect on handling. (And on performance in the case of rotational weight, but that's a separate issue.))