The main reason would be lack of interest. It's developed by a handful of people who are undertaking one of the biggest tasks in the history of the Internet with very little resources.
Remember how much energy it took to switch everyone to IPv6? Right it still hasn't happened 20 years later: that's exactly the kind of change GNS is about, and that's why it's designed to be backwards-compatible with DNS so we don't end up in the same situation.
I'm open to the idea that the provided implementation could be bad (haven't read the code), and i believe there may be valid criticisms of some parts of the protocol. I just haven't read anything of the sort so far. In both cases, that could hinder adoption, but both could be fixed before GNS is standardized, which is why the authors have been pushing early drafts to the IETF (to gather feedback).
> Are the prevailing registries/registrars opposed to it?
In regards to the ICANN, registries and registrars i'm not aware of any opposition. GNS changes nothing significant to their politics/economics: it would require some time/resources to deploy but arguably nothing terrifying. Registr* specifically are probably entirely unaware of this effort and couldn't care less, especially since GNS is designed in a backwards-compatible manner where people can continue to use regular DNS and it will "just work".
> Would there be any authority that could mediate disputes over who "owns" a name?
This is registry policy and has nothing to do with the protocol. Every registry has a different policy on this matter, which may also be impacted by local regulations.
> Does it address the problem of name-squatting/hijacking?
Hijacking is made harder by GNS because we use public-key cryptography from the start to delegate zones to a third-party, not location (IP/domain). It's like DNSSEC but better because you don't have to mess with your zone to advertise the key (the key *is* your zone identifier in the DHT).
As for name-squatting, this is again entirely unrelated to GNS as it has to do with ICANN/registry policies. GNS is about better technical foundations for DNS in the 21st century, it's *not* about changing the politics of the ICANN root (although it's arguably an empowering technology for alternative roots to emerge).
Hope i answered your questions. (there may be some details i got wrong as i'm not involved with GNS project)