Aaah. Yep. I remember that too, and I miss that. Dwarves vs Zombies was one among them that had managed to last for a while. I too have been annoyed at this issue.
It's hard to say exactly why we don't see many new original server types or gamemodes, but I don't think there's any one reason either. In order to stay up or justify staying up, a server needs players and that can be hard to come by. Many of the more massive servers these days got shot into popularity from Youtube. Additionally, some may be discouraged from trying to start their more original servers because of the existing lack of them, making for a bit of a vicious cycle. I imagine I'm missing several more factors.
Sometimes though there are niches that have to be filled, albeit probably with mods. The first thing that comes to mind for me there is something like a train or railroad server. Good luck trying to pull that off in vanilla where you cannot even connect minecarts.
The Meaning of Life, the Universe, and Everything.
Yeah, most of those servers are just exactly the same with no difference. That is why I play Hypixel, because it is still evolving around the interests of its players (except in Skyblock where the admins ruin your dreams lol). I just enjoy playing Hypixel Bedwars and Skyblock. Oh yeah. Speaking of Skyblock, Hypixel gave a very unique spinoff of it that is incredible.