Sometimes you only had upper case because the character generator was a 5*7 grid which wasn't really good enough to display lc.
replies(3):
The first one, there's preserved source code, I would know of, was by Ben Gurley (the designer of the PDP-1) and Weldon Clark, April 10, 1961. [0] This one featured just upper-case letters, figures and the very minimum of special characters. Notably, the input devices used featured provisions for upper and lower case, hinting at the programmers wanting something workable done quickly. Moreover, the character set may not have defined fully, yet, since (reportedly) Ed Fredkin, then at BBN (and mostly responsible for BBN acquiring the first production prototype), was involved in the final definition.
Later, the same year, we also see a fully developed font renderer [1], which uses the spare bit in the glyph definition to indicate a drop of the entire font matrix, which enables drawing lower case characters with descenders. (This convention was also used by the later hardware symbol generator Type 33.) With this, all characters can be displayed with perfect readability (with possible compromises only involving lower-case "f".) At the same time, we also see support for the full character set, including representations of non-printable characters for editing.
[0] https://masswerk.at/spacewar/chargen/pdp1-bbn37-crt.html
[1] https://masswerk.at/spacewar/chargen/pdp1-1961-codeword-crt....
[2] Overview with links to related glyph dumps: https://masswerk.at/spacewar/chargen/