Middleboxes like to drop any data they don't understand. No doubt there is one out there that is checking that all data on the dns port looks like dns and when encrypted dns is used it gets dropped. Using https for everything means middleboxes can't tell what is going on and just allow it.
DNSCurve is specifically designed to look like UDP DNS. The problem isn't that the packets don't validate as DNS, it's that some of the middleboxes modify them, or don't send them to the appropriate server at all and try to answer the query themselves. The protocol then detects that MitM attack and rejects the response -- as well it should, but detecting the attack doesn't get you to working DNS.
HTTPS can't save you from that, though, because the same networks that modify DNS queries to third party DNS servers also do things like require you to install a root certificate and then MitM your TLS connections, and drop connections that don't accept their root certificate.
In both cases it's the same thing. You can detect the attack but that doesn't get you through.
