Uppercase is numerically 65-90, lowercase is 97-122.
The numbers for uppercase can be represented in fewer bits.
"To implement a major change like lower case (keeping 6 bits per character in my syntax table instead of 5 bits) would have been a horrendous and risky job to do by hand." - Steve Wozniak
The early days of manual typewriters, paper tape, teletypes and vacuum tube systems, which already followed this practice predate US-ASCII, so I don't think the particular numeric values assigned by US-ASCII can have any explanatory power in answering this question.
Me, I blame the Romans ;-)
The Latin alphabet was initially only what we call the "upper case". What became the lower case came (a millennium?) later, first as an alternate style of handwriting and then as an addition to the alphabet along with rules about when which form of each letter should be used.
Given the need to economize as in 5-bit teletype codes it's not surprising the chosen convention was to print (or later display) those codes as upper case as that is, historically speaking, the default.
Still, I like to wonder if anyone every thought to build a teletype that printed in lowercase just to screw with people. :-D
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.
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:
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).
I remember this being a space-saving technique on TI calculators as well. Some of them allowed using lowercase letters in console output, but why would you bother when it ate into the KB of total storage you had to work with.
