The large SO reply missed a beat: unix was case-sensitive/lowercase preferred because Multics was. As far as I know, Multics was the first to be case sensitive. It had the advantage of being developed after ASCII had been defined, and by standardising on 7-bit ASCII. Other 36-bit machines of the era used multiple character sets, including SIXBIT, a six-bit character set with only one case, to pack an extra character in a word.