I suspect it's because the terrestrial planets can't hold onto large or many moons (Earth's moon is an anomaly, Martian moons are probably more common). That means that as orbits erode over time (like we see with Phobos), they will make thin rings that decay over the course of a few million years. On the other hand, gas giants can have and capture larger objects which will make rings with more material, probably also further displaced from the planet, if the rings are formed via collision, allowing them to last longer.