Because no one knows about it. But also, looking at their list of servers, the majority is offline. Assuming this is because any person can run one, it's probably the reason.
On top of the above, the likes of Google are enforcing their DNS servers. For example, I run AdGuard Home with it's DHCP functionality. I am unable to rely on the DHCP settings on my Android phone because Google forces the first DNS server to be 8.8.8.8 with (as far as I can tell) no way to disable this.
I think at least for the foreseeable future the solution is to run Unbound and stop relying on DNS resolvers combined with a firewall that redirects all DNS records AdGuard Home/PiHole. This is obviously not a solution to the general public.
On top of the above, the likes of Google are enforcing their DNS servers. For example, I run AdGuard Home with it's DHCP functionality. I am unable to rely on the DHCP settings on my Android phone because Google forces the first DNS server to be 8.8.8.8 with (as far as I can tell) no way to disable this.
I think at least for the foreseeable future the solution is to run Unbound and stop relying on DNS resolvers combined with a firewall that redirects all DNS records AdGuard Home/PiHole. This is obviously not a solution to the general public.