I think the problem with btrfs is that it entered the spotlight way to early. With Wayland there was time to work on a lot of the kinks before everyone started seriously switching.
On btrfs a bunch of people switched blindly and then lost data. This caused many to have a bad impression of btrfs. These days it is significantly better but because there was so much fear there is less attention paid to it and it is less widely used.
@lancalot I understand the list I provided is not necessarily complete, because Void & Solus are also independent, however, for them to be “main”, they should have “derivatives”, I don’t claim that I have a big Linux experience, but I tried & documented myself about the distros on the list, & can confirm that they are “main”, I also tried Nix OS, the use of 1 config file is refreshing, however that ease comes at the cost of some flexibility, installing Steam there is too complicated for me