Another signifficant factor is because the F-15 is an airforce aircraft. So, it doesn’t have to land on a carrier. Hence it can take plenty of runway length to land, which means it can touchdown at a higher speed. So, its wings can be permanently swept at relatively high angle, because there is no need to have heavy wing pivots that enable them to sweep forward and allow lower speed flight for carrier landings.
In comparison, the F-111 had a variable geometry wing so it could fly slower, when the wings were swept forward, because it was intended as a joint aircraft, and so there was going to be a Navy version, that had to be capable of landing (and takeing off!) on carriers.
The F-111 wing sweep is from 16 to 72.5 degrees. In comparison, the F-15 leading edge of 45 deg. The wing angle is asignificant factor that helps define the mach number at which the shock cone from the nose will impinge the wing tips. See below.
If the wing tips are outside the shock cone and in the sonic flow, the handling charteristics, as well as the increased drag, is likely to be very undesirable. The formula linking mach number and shock angle is given here by NASA.