Because there is no class that does what the warlord does.
At the unacknowledged 'role' layer, the Cleric, Bard, Druid & even Paladin all do what the Warlord did - they still have their "leader" capabilities, just more besides (the Paladin was only a secondary leader, and didn't exactly keep all his Defender toys, but that's another issue).
But, viewed that way, every class does stuff other classes do, just differently (or not so differently, but with different fluff (or with similar fluff, but different emphasis (OK, I'm still having trouble with Druid v Cleric v Paladin here

:sigh:

))).
Anyway, 4 classes do support, they all do it, in large part, with spells. There's not a precedent for scraping full-casting off a class with a sub-class. It'd seem an inelegant solution even by D&D standards.
A warlord subclass for each class would be neat thing to do, and I think lots of warlord fans would lke that, and those subclasses would likely also grab the interest of plenty of other people, too
...
a warlord class would be neat option to have, too, just like even though we have wizard subclasses for several classes (eldritch knight, arcane trickster, arcana domain, way of the 4 elements to name four ), it's neat to have a wizard class.
Doing both (/some/ of the former, anyway, not every class needs a sub-class of every other class) would be very much in keeping with 5e design - existing fractional-wizard sub-classes as you point out, and, for that matter, the Bladesinger on the other side of that coin - and, it would increase the chances of being able to choose some sort of Warlord-lite when dropping in on a random campaign, even if the 'controversial' full Warlord class were unavailable, you still might be able to play something better than a PDK. Which is kinda the point. As Mearls said, the Eldritch Knight is there so you can play a fighter/magic-user even if the DM is too young to get that reference, and didn't opt into MCing. OK, I'm paraphrasing, there.
