In WAB certain cavalry were rated as slower based on doctrine… poor mounts and simply lack of emphasis. Roman cavalry and Greek cavalry are given this treatment because their doctrine was to keep up with the line and support, more often than have a true cavalry offensive doctrine.
So it is a way to make Roman cavalry act like they did at Cannae--- where they immobilized themselves. Also it makes the Spartan cavalry at Lechaeum and Leuctra less effective.
That is why that happened. It started with AoA and hung on through the books--- it is debatable if it is a good way to simulate these deficiencies of Greek and Roman cavalry.. but it does design in such deficiencies without creating extra rules.
At Cannae the Roman cavalry was effectively "green" and trying to recover from heavy casualties in the previous three years.
For example one might argue that in a protracted war such as the Hannibal war that Scipio Africanus' cavalry would achieve a higher level of efficiency-- in that case they should simply be upgraded based on the scenario. Pyrrhus of Epirus had difficulty with Roman cavalry because they outnumbered his cavalry most of the time. The Roman cavalry suffered a major defeat in the 3rd Macedonian War at a time when they had more access to good horses….
There just are not that many occurrences when Roman cavalry achieved parity, so that is the reasoning and why the pain.
ponders nirvana.