It’s cause Epic/McKesson has complete control over the EMR world so everything has to work with them to some degree.
GNU health is great but I haven’t seen where it could support the massive amount of legal and monetary hoops that Epic and co have to jump through as well.
For some reason there just isn’t a lot of volunteer efforts/space for open source development in the healthcare world.
You could try Guix! It’s ostensibly source based but you can use precompiled binaries as well (using the substitute system)
It’s a source-first Functional package distro like Nix but uses Scheme to define everything from the packages to the way the init system (Shepherd) works.
It’s very different from other distros but between being functional, source-first, and having shepherd, I personally love it