What I think the biggest problem with the traditional package managers is that (1) they don't isolate packages from each other (when you install a program files are placed in many random places, like /usr/bin, /usr/lib etc) and (2) you can't have multiple versions of the same package installed at the same time.

This creates a lot of work for package maintainers who need to constantly keep packages up to date as dependencies are updated.

Also, because of this, every distro is essentially an insane dependency tree where changing even one small core package could break everything.

Because of this, backwards compatibility on Linux is terrible. If you need to run an older application which depends on older packages, your only choice is to download an older distro.

This is what snap and flatpak try to solve. I think they are not great solutions, because they ended up being an extra package manager next to the traditional package managers. Until we see a distro that uses flatpak or something similar exclusively, the problem is not solved.