I think the lack of monks stem from three issues: fantasy, history, and player reception.
Fantasy: The monk lacks a core class fantasy that is rooted in historic RPGs and fantasy worlds that predated WoW. Most people coming into the game are familiar with warriors, wizards, paladins, warlocks, necromancers, shamans, rangers, etc. We have most of the core archetypes covered in the game (excluding necromancy and summoner classes). A martial arts expert doesn't really 'fit' in the game. Monks are the least popular class in D&D in online polls and few other RPGs embrace the playstyle. It's hard to convince a player to embrace martial arts when you can play the standard archetypes.
History: Monks came out in MoP. Many people joined the game before that and had already invested in their mains and did not want to swap. Most people do not play dedicated alts either. DKs and DHs were an easier sell since they started so close to max level and had a much deeper fantasy and stronger roots in WoW lore. Especially back in MoP, leveling is not some trivial task. Even with the 100% exp buff, it can still take ~20-25 hours played to go from 1 to 120. And to be frank, leveling in WoW is not particularly fun for most players. Plus there is the ever growing wall of max-level catchup to do. If you rolled a monk today, it'd be 6-8 weeks of fairly dedicated play to catch up in terms of gear, corruptions, cloak level, etc. Back in Legion the wall was artifact level. WoD was an OK time to re-roll, but the leveling issue was still there.
Reception: Monks were unpopular at launch. I think this is largely due to them being so heavily linked to Pandaren and the 'kung-fu panda' meme at the time. Furthermore, Monks have been in a shaky state over the expansions. We've had monks for roughly eight years, and I'd say around 3-4 of them were good years. Most of the time it isn't obvious what they want monks to be and they feel overlooked due to their unpopularity. Monks used to be a very deep, rich class with many tools and options. Brewmaster used to be a very deep and challenging tank, with a large gap between the worst and best of the spec. Now it's fairly simple: take a cookie cutter talent build (there is almost no choice), keep up Ironskin Brew and use Purifying when brew capped (try to use a WeakAura to make this a bit more optimal). Windwalkers have an ability that forces them to hold still for several seconds, which can easily lead to unpreventable deaths, especially in M+ when abilities are hard to predict. Earlier in the expansion they had another ability that incentivized them to essentially take 100% of their health in damage in as short a time frame as possible (this damage is reflected back to the attacker). It was (and still is) very easy to die by accident as a windwalker.
FYI I main a Brewmaster Monk. I am sad that there are not more of us.