Ok, so they used the famous unit rather than the "historically accurate" one. Just like they used ragtag bands of ocean-crossing adventurers rather than the dreaded elite military of a nation for spain. (UU *should* be the tercio, not the conquistador, but the conquistador have more of a lasting fame these days, as does the greek fire, now matter how much history buff would like to think otherwise.
So in summation
-Firaxis won't put a unit that owe ALL its fame to historicla myth (Elephant).
-Firaxis is also apparently trying to swing wide of other games' choice of UU (namely Microsue's Age of series - hence not picking the little known cataphract ; the relatively not that known turtle ship and so on).
-Firaxis is finally trying to pick units that, while they do have some slight historicay justification (hence no elephants), are also easy to recognize as something meaningful for the players when possible.
-For a last note, look at the other knights replacement. They are all something basically VERY different from the knight. The Keshik and Ansar warrior are light cavalry type fellow. The Rider is a mounted archer. The Elephant uses a completely different (and much larger) type of mount that require a completely different strategy. And the samurai is not even mounted. They all have something that completely differentiate them from knight.
Comparatively, while Cataphracts were brilliant units, they were essentially just another form of heavily armored horsemen - which is to say, essentially, what the knight unit represent!
Frankly, considering how many good historically important civilizations didn'T even make the cut for the game, I think your attitude is more than a little of a "spoiled brat" behavior. BOTH of your favorite civs are in (Rome, Byzantine) whereas many of us have to deal with only one or none of them (I want Mali/Ghana/Songhai - one of them, not all three since they basically were much the same).
But again, I think that it is a good choice as well. 
