A lot of software isn't packaged for Debian. Especially complex ones and webapplications tend to be Docker containers or something like that. Home Assistant has a lot of Python dependencies which are a chore to maintain the Debian way. Same probably applies to some other distros. I mean it can be done, as Arch and NixOS show...
And you have Docker, you can install HA core in a Python virtual environment on any distro, or install Supervised, or the appliance (OS).
So there are many ways to install it. And I have the same complaint for other software. For example I'd like Nextcloud and a few other collaboration services to be available as distro packages. Sadly they aren't available like that.