The best answer is this…. It is angular momentum that plays a role but not quite as described. Because the planet is spinning, it creates a non perfect spherical shape. To put it simply, the area around the equator moves fastest as the planet rotates, and it is this area where more material converges, creating a bulged, subtle saucer or egg like shape. Because of this convergence, gravitational force over the equator is greater than say over the poles. This is an oversimplification, but it’s simple and easier to understand. To elaborate, gravity can be generalized into a single vector or direction as with Newtonian mechanics. The same idea is indoctrinated into relativity, but is expanded on for particular directions (tensor products) which average out to an overall force in a single direction. When considering accretion of rings, you must consider the relativity point of view. The overall force may have a single direction, but this way of thinking is too narrow to describe what happens when rings form. If you consider that the direction towards the equator is greatest, perhaps the same notion that discerns Newtonian mechanics can be used to intuitively understand that the overall force will draw things towards the equatorial plane. More of the planets mass is concentrated there, hence gravity drawing more there. Give a system enough time to evolve and material will collect there.