I believe that the Nextcloud team have said that, for the time being, they will not be doing packaging and distribution for Nextcloud. They hope that the distributions will pick it up themselves, because it was too much work last time, taking the team’s time away from developing the core platform itself. Nextcloud is pretty easy to install on Debian as compared to using a repo (or .deb) - all you do different is that you have to manually extract to your /var/www directory and edit the config files yourself.