Limiting support of software to specific software configurations makes sense.
Its stuff like Debian might be using Python 3.8 Ubuntu Python 3.9, OpenSuse Python 3.9, etc.. Your application might use a Python 3.9 requiring library and act odd on 3.8 but fine on 3.7, etc.. so only supporting X distributions let you make the test/QA process sane.
This is also why Docker/Flatpack exist since you can define all of this.
However the normal mix is RHEL/Suse/Ubuntu because those target businesses and your target market will most likely be running one.