Put simply, mount is when you are directly on top of your partner and side control (AKA cross body) is when you are perpendicular (but on top) of your partner. When it comes to submission grappling, which is the better position for the top player?
Supposedly it’s mount. Truthfully, when one starts, it’s often side control. Why does this disparity happen? It’s hard to say exactly but I suspect there are at least two factors. The first is that students don’t know how to drive their weight down effectively and the second is that students don’t know many submissions from the top.
If those two points do indeed constitute the bulk of the problem, the solution is simple: practice holding top and practice submissions from the top. (Once again, practice is the solution.)