I think it boils down to demand. The job of Cataphract and Cavalry can be filled by other units and be just as effective with in less time.

To make a Cataphract effective, get Ironworking to 10, Military Tradition to 10, Horseback riding to 10 (this requires a Stable to 10 as well)
Alternative, Archery 10 and you range them and make 400% as many. 9 for an Archer, 35 food for a Cataphract. It's what makes Swordsmen a seldom used unit as well. Benefit doesn't outweigh complete cost.

Perhaps later, they would be a prestige unit using them over the lesser but the game is too much about hard numbers and the volume reduction makes prestige armies unlikely.