The reason is probably because Cataphract becomes insignificant with its Acrobatic Mount and Trot.
You got Initiate, which is a huge damage boost if you have got a strong weapon, you have got the Dragoon Helmet, which boosts all spear damage by 50%, Gungho, which boosts damage by 40.6% with Boruta Seal, Dragon Fear, which boosts all damage by 15%, Sharp Spear, which boosts all spear final critical damage by 41.6%, Spear Lunge, which boosts spear damage by 50% (iirc,might have changed).
That’s way more than 100% damage boost already, meaning adding 64% of Acrobatic mount can shrink to less than 25% actual overall damage boost, but in return you get crappy SFR skills, so even the slow debuff that doubles Dragoon skill damage doesn’t help much here.
You also shouldn’t forget that Lancer adds a lot of mobility via skills and has pretty low cooldown times for its major damage skills on top of it.
Hoplite focuses on both Dragoon and Lancer, it boosts the final critical damage by over 40%(which is what makes Blossom Blader so strong for example), it is the only spear class with a crit rate buff, it is the only class with a strong spear debuff and has the strongest damage skill spear has to offer in combination with Dragoon Helmet (Pierce, up to 52290% against bosses without the vaivora weapon before any damage boosts).
If you take Cataphract, your focus wanders off to Dragoon, forcing you to steed charge every time to double the damage you deal with Dragoon skills to somehow catch up to what you could’ve done with Hoplite instead. Cataphract is more focused on PVP with Trot and Impaler.