Because .onion isn't a DNS domain. It uses the same syntax but you have to be on Tor for it to work and it does something completely different than DNS resoluti

Because .onion isn't a DNS domain. It uses the same syntax but you have to be on Tor for it to work and it does something completely different than DNS resolution. I read the original question as specifically about having top-level domains, not "something like domains that works on a special network". Special networks can do as they like, but then they won't be DNS.