Games that require an ounce of strategy or teamwork always die. Look at UHC -- it's a game where every move you make matters, every little bit of your health precious. No one played it though, because it's a game that heavily discourages rushing into a fight and rewards players that use alternative combat styles like bows and fire. Our players hate bows. They just want to click each other until they die.
Blockwars is fun. It rewards action over inaction (a big problem that faces island games) it's fast-paced, exciting, and *usually* doesn't drag on for too long. Sadly though, if you're not with a group of friends on Discord and you're facing a team that's all talking to each other, you're at a huge disadvantage. This is why I almost never play games like Blockwars, Archer Assault, Team Anything, etc. without a team. It's just not fun to carry a bunch of rando's.
Also, in general, it seems that Minecraft PvP is getting incredibly stale. It's been the same for years, and the 1.9 update barely changed it if you think about it. You still click things until they die; now you just have to do it a little more slowly. I like non-traditional PvP, like Archer Assault, Battle Zone, and certain parts of Lucky Islands. Giving the player more agency and freedom in a game makes it a lot more fun, and frankly, sticking a sword in someone's hand and telling them to go repeatedly click their enemies until they die is not agency; it's a chore. Agency and freedom is what makes Slime Survival good; you're fast, you can teleport and leap and do all sorts of crazy stuff, and even as the slime, you can make it bigger, smaller, shoot faster, throw a grenade, etc. It's not just a matter of clicking people.
In short, Blockwars is dying because team-based games are no fun if your team is bad, and Minecraft PvP needs new, better mechanics.