The fact that there is the POSIX standard and multiple shells that meet that standard is also a big part of it. If you write in a POSIX compatible shell languag

The fact that there is the POSIX standard and multiple shells that meet that standard is also a big part of it. If you write in a POSIX compatible shell language you know it can run anywhere. Fish, for example, has no standard and only 1 implementation. Any new shell that hopes to compete in the systems space will have to be POSIX compliant. At least until there is a new widely adopted shell standard.