(If you look at the other popular games on Twitch, they all provide servers and can't self host. GTA V, Fortnite, LoL, CoD, Valorant, etc. So there is no market for anything but Minecraft-related services.)
In addition to its popularity, I would guess that this is probably related to the fact that the average age of Minecraft players is probably lower than the average age for most other popular online games. A disgruntled person between the age of 12 - 18 who knows they can completely shut down the fun-having ability of everyone they're pissed off at for a few dollars per hour will often feel pretty tempted.
If an enterprise customer will pay $$$ to support X this can still make financial sense, but Cloudflare's non-enterprise customers aren't paying $$$.
Minecraft is apparently not going anywhere, it's still very popular a decade after release. And my understanding is that the protocol is fundamentally the same as ever. So, you do that work once, and then you've got a free proof of concept apparently forever.
I mean, I guess it's a compelling use case for some customers. Still, it's a weird outlier.