They fullfil different niches. Elementary is laser focused on macOS converts, pop is focused on gamers + enthusiasts, etc. Ubuntu at its core is a decent operating system, the distributions just make it easier to get an out of the box config that "just works" for your use cases.
Well, Ubuntu is a great base (like all the commands on github repos are for Ubuntu). But Ubuntu isn't such a great distribution. So everybody is trying to fix it by pulling their own distro :)