This might give you a clue of its popularity: https://us.forums.blizzard.com/en/wow/t/rp-pvp-realms-reconsider-one-realm/130723. In short, the answer is that it’s pretty popular.
So the obvious question is: why did the server die? It is not as an easy question to answer compared to the one you posed. There’s been (to my knowledge) no investigation into this and the interest in doing such an investigation is probably fairly low. So I’ll give you my own speculation based on what I’ve experienced and what I’ve discussed with others backed up by what few sources I can find.
RP-pvp servers do attract a certain type of player. I am one such player! And I was pretty adamant that I wanted to play on an RP-pvp server and I had a few friends who didn’t really have any preference one way or another. So I convinced some of those people to try the game with me, and I likely wasn’t the only one doing so. My point being: not everyone who plays on an RP-pvp server is necessarily there because it’s an RP-pvp server.
Zandalar Tribe had a pretty decent population throughout Classic and even throughout Phase 1 and 2 of TBCC. The statistics over on Ironforge show this:
https://ironforge.pro/population/archive/ZandalarTribe/
https://ironforge.pro/population/tbc/ZandalarTribe/
(although keep in mind that Ironforge statistics are not wholly representative of a server’s population or activity)
If you look at the server stats in the second link you can see that the activity is certainly lower than it was during the first months (which should have been expected) but it’s still pretty active. It wasn’t worse than it had ever been in original Classic. So what caused the dip in activity? There are a number of plausible causes. New World, a shiny new MMO, was released the 28th of September. Season of Mastery began the 19th of November. And, what I believe to be the most damning of all, Blizzard announced the first batch of free server transfers the 10th of November. Zandalar Tribe is on that list now but it wasn’t back then – but the narrative that the server was dead caught on fire shortly after the free transfers began.
The main arguments (as to why the server was dead) I saw was that it took too long to find groups for heroic dungeons (or that it was “impossible”), that there were no people to group up with while levelling alts and that raiding guilds didn’t have a large enough pool of players to recruit from. The people pushing these arguments all wanted more players on the server for their own convenience and believed the grass to be greener on the other side which it may have been to some but probably not to all. The first mistake Blizzard did, in my view, was to allow free transfers from Zandalar Tribe.
The reason Zandalar Tribe is in the state it is in is because Blizzard allowed it. Because as I said at the start, not everyone who plays on an RP-pvp server because of the RP-pvp tag, and those people likely would have had fewer doubts in abandoning the server. So what about the people who do care? Well… staying whilst others abandoned the server en masse imposed a certain risk. Many likely reasoned that they did not have a choice in the matter, and so were forced off the server.
And this is where the Ironforge numbers are so treacherous. Because what about players like me? I was part of a fairly casual guild that did some raiding (for the record we did not log our raids and so our activity did not show up on ironforge), but raiding was not the main focus of the guild per se. The community was. Yet we tried to organize 25man raids and we had some success – early on we teamed up with a similarly sized guild to take on Gruul, and a large chunk of this guild left the game to go and play New World. We then organized pugs for some time and we were actually successful in doing so. But we could only ever keep doing T4 content. We could not pug T5, and we failed to find someone to team up with. We decided that we did not want to sacrifice the soul of the guild by rebranding as a raiding guild, and so most of us went into hibernation. We stopped playing.
… and now with Zul’aman and Magister’s Terrace out, some of us have slowly crept back. Some who left the server have come back, even though they have been unable to transfer their characters here as that has been blocked by Blizzard! They have instead chosen to roll new characters. The resurgence isn’t massive by any stretch of the imagination, but it is real. What if Blizzard had never offered transfers off the server in the first place? That’s, sadly, a hypothesis we cannot really answer. But the consequence of the reality we’re in may very well be that there won’t be an RP-pvp server in Classic anymore. As for why that is a problem I’ll refer you to the first link at the very start of my long, long post.