People don't know what they want package mangers to do and they had much less of a clue back when the fragmentation started. So we got rpm and deb. Arch decided these were too complicated when you wanted a rolling release so they have their own package manager. Programming languages need to have their package managers work on more than just Linux, so they make their own cross platform ones, but since the devs are only thinking about their own language they don't make something cross language.