[chasil]

The teletypes needed all the control characters as well (cr, lf, bell, etc.), so everything below ASCII 97 was mostly required.

If I remember correctly, Apple terminal emulators set reverse video when they meant a capital letter, so you could converse with something over a modem that was sending lowercase.

[mannschott]

When I wrote "teletype" I wasn't referring to what teletypes had become by the time by the time Unix was developed. Think further back than that. This is why I mentioned 5-bit codes:

https://en.wikipedia.org/wiki/Baudot_code

This coding scheme is so constrained (32 possible 5-bit values) that it uses the codes FIGURES (01000) and LETTERS (10000) to toggle back and forth between two alternate sets of meanings for the remaining 30 possible codes. Still not enough space for lower case or ASCII's plethora of control codes (just NUL and DEL).