It may have something to do with Apple File System. So far, that system is case-sensitive, whereas HFS+ is case-insensitive by default. Apple has not disclosed whether they will keep this as the default in the future. A lowercase username is simply conventional on Unix systems, especially on file systems that are case-sensitive.
Not only conventional it is mandatory unless you go your of your way to force it despite the multi line warning explaining why it a bad idea to do it when using adduser or useradd on a *nix system. Shocking as well after all these years they finally did the right thing and went case sensitive on file names. No more warnings that This is the same as this when renaming in the GUI.