Because the routers can come back, there's always a temptation to try to wipe them out with your cav. When you're grossly outnumbered, don't succumb to the temptation; keep the cavalry moving and routing more units, not finishing off the ones that are already running. Even if they re-form, their morale will be crappy and they won't be able to take another serious charge. And if you can get everyone routing at once, you win.