It's not that the warlord couldn't be a sub-class. It just can't be a sub-class of the current 5e fighter.
Illusionist fit into a wizard, because spells are modular. You can readily swap an evoker feature for an illusionione feature. i.e. you lose burning hands, and gain silent image. The current 5e fighter can't lose multi-attack to gain inspiring word. I did attempt a warlord sub-class that swapped multi-attack for maneuvers, but it was very clunky, and you still had multi-attack.
The warlord could, for instance, have easily been a part of the playtest fighter (superiority dice and maneuvers). Just by making warlord maneuvers.