This effect has nothing to do with Hill spaces. There are stable Lagrangian points 60 degrees ahead of and behind Tethys and Dione. These points are where gravitational pull inward exactly matches the outward centripetal force on the little Trojan moon that’s going way too fast for its weight class.
So what happened to other moons that weren’t in a stable Lagrangian point? They probably escaped or collided with each other, adding material to Saturn’s rings.