Most UNIX and UNIX-like system (And my current prefered OS, Plan 9 (Which is more UNIX than UNIX)) differ between small and capital letters, meaning, that having your username in capital letters, or just one capital letter, would make you having to press an extra button.
UNIX is a trademark name for UNIX and UNIX certified systems(Like Apple's OS X). The capital letters are part of it.
Furthermore, i must admit that the fact that my username consists of 3 letters and all are small letters is also a tribute to the people i look up to, like; dmr, ken and bwk.