If you nerf horse speed globally, you'd make the courser the only viable horse. If you nerf maneuverability globally, you'd make the Arabian Warhorse (or possibly the Steppe too) the only viable horse.
Maneuverability and speed are inter-related. You can't reduce one without affecting the other. Taking all horse's speed down, for example, would make all horses useless, except for the courser, which would be benefited (relatively, compared to the other horses) by the lower speed but same maneuverability.
Also, a lot of these suggested cav fixes don't take into account that 'cav' is quite a mixed class. Sure, 90% of cav players are lancers, but most of the suggestions I've seen would barely hurt lancers and instead hit sword&board, 2hand, and possibly horse archer cav much worse.