The problem is, in regards to main tanking, is that both warrior and paladin have such innate tankiness, which drk is missing.

War have parry up + hp up + healing up in 1 move essentially.
Paladin have damage down + natural shield, so they are very tanky too.

Both will make great main tanks AND off tanks (while pld is a better progression tank, it can definitely be done with a warrior these days.)

Dark Knight just lacks the same innate tankiness when main tanking.