That question wouldn’t have occurred to me, but it did to the French philosopher Montesquieu in his analysis of the decline of Rome. He’s no slouch. He probably was one of the two or three writers who had the most influence on the thinking of the Founding Fathers.
So here is what he says:
“When the Romans were in decline, they had almost nothing but cavalry. It seems to me that the more expert a nation becomes in the military art, the more it makes use of infantry and that the less it knows of that art, the more it enlarges its cavalry. . . . In short, the force of cavalry is momentary, whereas infantry acts for a longer time; but this requires discipline.”
I dunno. Sounds to me like he is just riffing on one example. Me, I think that having lots of cavalries often is generally a sign of national wealth—for each cavalryman, you need a horse or two, plus a couple of helpers. That’s a lot of mouths to feed.
But being cavalry-heavy can also reflect the circumstances of the time and environment. For example, by the end of the Boer War, the Boers were mainly a mounted force. I think the same is true of the Comanche nation at the time of its greatest strength when it could bring to bear more firepower than the U.S. Army could.