One thing no one seems to have touched on yet: distros have philosophies—guiding principles that affect what packages they have and how they’re presented.
For instance, Debian is strongly open-source-centric. Closed-source software is not normally found in their main repository (even when it would be useful to put it there, like some drivers).
Gentoo, on the other hand, is all about user choice. You’re expected to choose for yourself whether you want to use systemd or OpenRC, X or Wayland, which DE (if any) you want to use, and which features you do or don’t want compiled into your software. However, Gentoo is quite happy to include closed-source software in the main package repository, because using it is also a choice that some people prefer to make.
Red Hat, Arch, and Slackware (to name the remaining major foundational distros) also have their own philosophies. Some descendant distros retain their parents’ principles, while others discard them and develop a philosophy of their own (Ubuntu doesn’t have Debian’s Open Source Uber Alles tendencies, for instance).