Pikes are pushed against/into the ground, so neither morale nor momentum will do you any good against them. Discomforting the pikemen will also not do anything, since pikes and their use in formations are designed for that to happen. If you get charged by a fuckton of cavalry, you are bound to flinch anyway and you weren't supposed to actually do anything with the pike but push it into the ground if you were charged. The momentum is provided by courtesy of the attacker. So the downhill movement and the momentum are a disadvantage against pikes.

And I'm not even counting the fact that horses will not charge walls of spiky things. They are living beings with a mind of their own. A fact that we tend to forget nowadays. But I'm willing to let that one slide, perhaps there was some super special sauce training only known to the horse freaks of Rohan that made them go through with the charge.