Besides what James said regarding existing population problems, a few things could be added. There were many balance changes over the years that made several classes either require extreme dedication/skill to play, or made them simply unplayable (horse thrower anyone ?). There is also the problem of levelling, meaning old players, who are already far more skilled at the game on average, also happen to have full sets of loomed items, and more often than not, a level 35 character at the very least. Imagine someone new hopping on the game, wanting to play crossbow or thrower, and being faced with a level 38/39 player : they aren't going to have a good time, so they leave the game and don't come back; simply because they were faced with an impossible challenge, which didn't end up being fun for them.
cRPG didn't know what it wanted to be years ago, so many balance changes, justified or unjustified mind you, made a lot of people leave the games in waves. One of the reasons mercs is appealing is because of the cheese, which means there are a lot of skill equalizers, just like in native. Add to that cRPG is now limited to DTV, slaying the same waves of bots using the same tactics every time (fence, hut, staircase, cav kite, shieldwalls); and on a good day, small-scale battle with hardcore, dedicated players, which leads back to what I was talking about earlier. I'm personally fine with cRPG being "competitive" in nature, but it needs to change to attract a fresh playerbase, which is difficult itself because warband is slowly dying now that bannerlord released (and failed).
There is no silver bullet that will revitalize cRPG, however some solutions can be thought of : one may lie in making nearly all builds possible again (the way it was pre-Tydeus); perhaps possibly using the strat item factions to create DTV bot factions factions, and dedicated bot trees for said factions, where there would be a dominance in low armor enemies (the way warfare mostly was back in the middle ages), with a small core of "elite" enemies to kill, growing wave after wave. Perhaps the bots could use some AI leader to order the troops against the players, and that could shift the PvE element from defending the virgin to focusing on player survival only. Regarding battle, banner balance needs to be changed to include class as a factor (one of the few good features of merc is that your character has a class attached to it depending on its build). Finally, the reward system needs to change, and the XP grind needs to be reduced, and capped, because despite being a core element of old cRPG, it now only creates a gigantic gap in levels for new players, and makes this gap very hard to breach for people who have a life. Restoring strat is a good idea, but without players to play it, I do not see the point. Furthermore, I think the 6 loomed items players get should be given with a few free exchanges so they could choose to allocate their points on other items. Finally, I think there needs to be an up to date guide for all the mechanics of the mod, where all the balance changes compared to native are noted, or at least the most important ones, because players are often completely clueless and might be tempted to make builds that do not work at all, adding to the frustration.
The post is disorderly, but I think it is fairly comprehensive. This isn't a diss to the current devs who stepped up by the way, more of a critique towards the way it was managed up to 2018. Whether they can do anything to revive the corpse for the 27th time or not, that I do not know.