The computers my mother worked with (1960s?) only had 6 bits per character. So with 2^6 = 64 different characters, there weren't enough characters to have both upper case (26 chars) and lower case (26 chars) plus all the numbers (10 chars) and symbols etc. you'd need. So they only had upper case.

I think it sort of stuck from there, that computers and commands etc. used upper case.