Git was built specifically to scratch the itch of Linux kernel developers.
Linux kernel developers tend to like making sure the Linux kernel works on as many obscure architectures as possible. See https://en.wikipedia.org/wiki/List_of_Linux_supported_archit...
Making sure Git is portable across system architectures is quite important.
Also, portability in the "being available to many systems" thing is important for a lot of developers. I build stuff that currently has to deploy on SPARC/Solaris, but there are plans to make it so that in the not-so-distant future, all that stuff will be moving to virtualized clusters of x86_64 Linux. Portability in the narrow UNIX sense is pretty damn important to a lot of people.