Because Homebrew is unfortunately "the standard" and the first place a project will package for if availability for macOS is desired. As a Linux user, it's the worst package manager I've ever experienced but it is what everyone is using in my company who uses macOS (most people).
"pull requests for new packages are welcome" is the self-defeating answer: if you as a user have to create a PR for a package to be in MacPorts then it means that MacPorts have not attracted enough attention from the authors of the package to submit it themselves.
In the Nix world, at least, packages are often added by people who want to use them. I think that's more common than the fulfillment of a packaging request.
It would be funny if the state of packaging on Linux would not be so pathetic.
If authors care about Windows they do produce Windows installers and/or publish it in their store.
If authors care about macOS, they produce .dmg or publish the app via Mac Store. Additionally they often publish it on Homebrew.
If authors care about Linux, they provide packages for Ubuntu and Fedora, and sometimes for more distros from the long tail (usually Debian comes 3rd as it is simple to add after Ubuntu).
It's so slow: on the order of single digit minutes to a dozen to install a single package. It feels messy, like it's shotgunning my entire system with bits of itself (I don't know how true the reality of this is). I've not infrequently had packages simply fail to finally resolve after they did their dance of resolution. It does not inspire delight. The vibe, if you'll excuse me, is very crude cudgel.
Oh, the install location it asks you about when installing Linuxbrew is extremely odd. One choice offered is to make a dedicated brew user directory and then install there? That's terrible practice. No one else does this because it's terrible. And then the other apparently breaks a bunch of stuff and is not recommended.
