a minimum wheelbase vehicle (rules limit you to 60 inches) allows you to build the smallest (and therefore lightest) possible chassis. A longer wheelbase has better longitudinal load transfer characteristics. On a typical AutoX style course, longer wheelbase is faster if you neglect the adverse effect on mass and yaw inertia. Additionally, short wheelbase is better for accel, where you want as much load as possible on the rear wheels. More critical for the big four cylinders, as they are more limited by traction.
There's more to it than that, though.