Linux has standards where interoperability is important. The more things needs to talk to each other the more they need a common standard to talk over. Things like X11/Wayland don’t have many alternatives as so many things need to talk over them. The only reason there are two standards here is because X11 has massive limitations that cannot easily be worked around.
For package managers applications don’t care about them. Interoperability only matters within a single distro. So people are more free to create what ever standards they want for their own distros. And when people can choose people have opinions and these opinions evolve over time. Which results in multiple competing products that effectively do the same thing.
And here is my hipotesis if the GNU project came up with a good and easy to work package manager in the early days of Linux
Probably, but creating a good, easy to work, fast and reliable package manager that meets everyones needs when you are discovering how you want it to work for the first time is extremely hard. And even if you created a perfect one at the start, requirements can change. This happened with X11, and even with package managers seeing the rise of things like flatpack, snap and appimage that all work fundamentally different from the traditional ones.