Because thos distros suck for any kind of real life use case. If you want a working OS with for gaming, your office job or just regular browsing, then this ain't it chief. If you want to have a project you have to tinker with every day, then sure, go for it! But most people don't want to be bothered by their OS.
Back when I used to use Arch (btw), I had the bright idea of switching to Parabola because 99% of what I used was open source, only to find out that there's a hidden 5th freedom that's required for something to be classified as truly free software: Stallman's freedom to have his balls fondled. It tried to uninstall half of my packages because of ideological stupidity such as the software having optional closed source add-ons even though the part that I had installed was 100% open source. These distros are useless toys.
These distros reject everything that is not free as in free speech. This means no binary drivers, no binary firmware, no binary software. While this is very idealistic, not in a bad way, it might be impractical for most people. Start with an "easy" Linux, you can always go the hardcore way afterwards.
The idiocy goes beyond rejecting non-free software. For example, Debian ticks all the boxes needed to be added to that list but it's not due to the fact that they dare to have a repository with non-free software that isn't even included in the OS.