If you have a true melting pot, it works, but you have to have a common culture into which all groups assimilate. When you have, as we do, increasing numbers of subgroups that not only refuse to assimilate, but actively demand that the rest of society accommodate them, it doesn't.In our world that is so interconnected, it is difficult not to have diversity. But it takes work to make it work.
It is easier to just say, I will not accept others not like me.