Maybe it wasn't the amount of horses but the kind of horses that the Romans bred in Italy. Maybe compared to the larger sturdier horses of Numidia, Africa and Spain, Italian horses were no match for them.

Or a thought I have just had, maybe even at this early stage the equites were getting used to the new wealth they were enjoying under the infant Roman Empire and did not want to take the time to learn to be an effective cavalry unit. As for the allied cavalry, surely they spent most of their time farming their land instead of being drilled to be a decent cavalry force