It's a shame that snaps are forced to use Canonicals closed source backend because they are really good, and a fully snap system is a very compelling idea for immutable systems
They're not forced to do so. You can install snaps locally (or provide a distribution system that treats snapd much the way apt treats dpkg), or you can point snapd at a different store. The snap store API is open and documented, and for a while there was even a separate snap store project. It seems to have died out because despite people's contention about Canonical's snap store, they didn't actually actually want to run their own snap stores.
I don't know why you're getting downvoted. It makes perfect sense that Cannonical made it's own proprietary package ecosystem and while technically anyone can build their own snap store, ain't nobody got time for that.
It's Cannonical. They prefer implementing everything themselves fast, rather than developing a more sustainable project with the rest of the community over a longer timescale. When they do that, there will be very little buy-in from the wider community.
Others could technically implement another snap store for their own distro, but they'd have to build a lot of the backend that Cannonical didn't release. It's easier to use Flatpak or AppImage or whatever rather than hitch themselves onto Cannonicals's homegrown solution that might get abandoned down the line like Mir or Ubuntu Touch.
I don't agree that it made any sense to do that. If they wanted to containerize apps, there has been an open source solution to that for years; Flatpak.
ain't nobody got time for that
As an app maintainer, that wants to support Ubuntu, why would I prefer to deploy a snap server, instead of publishing deb files, or creating a Flatpak?
It's Cannonical. They prefer implementing everything themselves fast, rather than developing a more sustainable project with the rest of the community over a longer timescale. It makes sense that when they do that, there will be very little buy-in from the wider community. Much like Unity and Mir.
As you say - why would others put time into the less supported system? Better alternatives exist. If Canonical want their own software ecosystem, they'll have to maintain it themselves. Which, based on Mir and Ubuntu Touch, they don't have a good track record of.