Popping by to add my 7 cents:
Given that the number of classes exploded in 3 and 4, and now we only have a handful, I have to wonder where the zealous, uncompromising defenders of those other missing classes are? Sure, here and there is somebody looking for a Hexblade or a Runepriest or a...something (look I'm making this part up because I don't know what those classes are) but nowhere is there a relentless drumbeat of demand that comes anywhere close to that for the Warlord.
You ask a good question. Where are these fans? There are likely several factors at play here, some of which requires pointing out the obvious.
1) Not all classes are equally popular or have fanbases. How or why that is the case is a separate subject entirely. So when we take the Hexblade or Runepriest, following your example, we may ask how popular these classes were in the context of 4E (or potentially in their pre-4E roots). I doubt that many 4E fans would dispute, for example, that the Seeker was something of an unpopular or dud of a class in the context of 4E. So the demand for updates will not be equal across classes.
2) A number of these other classes had "good enough" updates to 5e. These other classes may have received serviceable updates that integrated them into other classes. The subclass system works for creating variation of playstyles
within classes, largely reducing the redundancy of other classes. The Assassin and the Illusionist, for example, were both turned into subclasses for the Rogue and Wizard, respectfully. And, for that matter, a number of popular 3E Prestige Classes were also turned into 5E Subclasses (e.g. Dragon Acolyte, Arcane Trickster, Assassin, Lore Master, Shadow Dancer, etc.). We also see this with the 4E Invoker or Avenger. Many Invoker players have simply shifted over to the Light Cleric. The Invoker was a "divine blaster," and the Light Cleric through its subclass features and spell choices allows for a player to sufficiently mimic that niche. The Favored Soul will also likely cater towards those ends. Many Avenger players have shifted over to the Vengeance Paladin, though there are still the occasional calls for a divine subclass of Rogue that fills a similar niche. The Hexblade is now a subclass of the Warlock, as of the most recent UA, which seemed to be well received.
But...
3) Not all class updates to 5E are "equal" in their authenticity or faithfulness. This is where, for example, I suspect we see some demands for other favorites of 4E or older editions. The Shaman was a fairly popular class in 4E. People have expressed desire for a Shaman class or subclass. The Circle of the Shepherds in UA was likely an attempt to re-create a Shaman/Summoner subclass for 5E, though it is questionable how well it succeeded on that front. And although we have a number of ways of creating a "gish" in 5E (e.g. Eldritch Knight, Valor Bard, Bladelock, Paladin/Sorcerer MC, etc.), we can also see that the Swordmage has some demand as well. And we also saw a recent UA attempt to give the Stone Sorcerer the Swordmage's stuff. But we also saw the first iteration of the Artificer on UA as a Wizard subclass. This received a tremendous amount of negative feedback, hence it being made into a full class in a recent UA. This decision received much better positive feedback. This has moved the UA Artificer much closer to the realm of Point #2. If the Artificer becomes "good enough" then the clamoring for its inclusion will likely stop. Basic goals achieved.
So when we look at the question of fanbases and edition updates, it's of course worth asking how the Warlord situates itself in this conversation. For those oppose a Warlord class, they may feel, in part, that the Warlord does not satisfy #1, which has been a source of nebulous debate. How popular was the Warlord class? How big is its player fanbase? (Sadly a number of the old polls about class popularity have since been removed from WotC's website.) Or they may believe that existing options (e.g. Battlemaster, Mastermind, Purple Dragon Knight) satisfy #2. So they may not understand why Warlord fans want a full-class update when they believe "good enough" options already exist
for their purposes. Naturally, advocates for a full-class Warlord archetype see the issues differently. In this case, they may regard the Warlord as having a sufficiently large fanbase to warrant its inclusion as a class. And in regards to Points #2 and #3, Warlord fans do not see the available options as being "good enough" to fill in the unique playstyle niche that the Warlord (and its spiritual predecessors) filled in terms of martial tactical support play. Here, many Warlord fans cites the Battlemaster or Purple Dragon Knight as being as much of a Warlord as the Eldritch Knight is a Wizard or the GOO Warlock as a Psion. I also think that the comparison to the Artificer is now warranted given how it originally started as a Wizard subclass before being expanded into its own proper full class. The "zealous" calls for a Warlord class are still around because the absence of their unique niche is still felt in 5E (i.e. Point #3).
