Portage is cute but it's tied to Gentoo and few people want to compile their own packages.
Apt was one of the earliest package managers for Linux, for a popular distribution. That was enough to give it a head start regarding packages available. And that's all you need, unless the other solutions are obviously superior from the user perspective, which they're not.
Strongly agree. Gentoo's Portage is the best experience I've had with a package manager on Linux.
My package management observations, as someone who's been using them for... a while, now.
Best experiences: Portage on Gentoo (lots of options; many packages; fast; reliable unless I broke it doing something stupid; maintaining a high level of control over the system at a low level is the only way I've ever managed to make Linux behave itself), Homebrew on MacOS (great package selection; casks are wonderful; outstanding CLI; keeping user packages away from the core system has really grown on me)
Middling: apt on Ubuntu and Debian (OK but increasingly feels dirty after getting used to Homebrew and keeping my user packages separate from the system; breaks "on its own" more often than Portage did, though still rarely; package selection just OK)
Bad: Yum/RPM on Red Hat, Fedora, and long ago, RPM on Mandrake (sloooooooowwwwwwwww and RPMs seemed to break constantly back when I used them), Macports on MacOS (slow; broke so bad the easiest thing to do was to delete the whole dir and start over about once a quarter; limited package selection)
Incidentally, Gentoo's OpenRC is the only Linux init system I've ever used that I liked and felt like I actually understood well enough to use it without apprehension or constantly needing to check documentation.
I like the idea of Nix and tried NixOS once but after an hour or so of following a guide and troubleshooting couldn't get X running so gave it up as "for those with more free time than me—try again in a few years".
I try about 1-2 other distros every year (in VMs or a spare laptop), and I always stay with Gentoo. I won't go so far as to say that it's objectively better, but after 10 years or so just nothing else compares for me.
Once I needed a package from an external repository, apt has almost always fallen down into a pit of duplicate and incompatible dependencies. Maybe it’s better now, but I was burned long ago and never looked back.