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.
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.