A lot of reasons, some are dictated by the rules of the organization, some have an overall max width, others have a tread width, plus you can change the circumference by changing wheel width, also the tire manufacturers have a recommended wheel width for a certain size tire, plus a lot is driver preference, and the feel they get with a wheel size, as it also changes the stiffness of the sidewall, plus the tread patch changes, so, there are several effects, it is hard to make a statement that covers everything. Bob!