probably to make sensible use of limited display abilities of early hardware, fitting characters in 8x8 boxes and making it readable on blurry fat beam glowing green screens quickly becomes a problem with lowercase (size, readability, ascenders, descenders, baseline), whereas uppercase is comparatively easy.
early hardware didn't have pixels or, except for debugging, screens; it had typebars which slammed bits of type into an inked ribbon and the paper. we're talking about 01930s to 01970s here. the baudot code didn't have separate codepoints for upper and lower case. in point of fact, it didn't have separate codepoints for numbers ('figures') and letters either; it had shift characters called 'figures' and 'letters'.
but why uppercase rather than lowercase, you might ask? well, http://wiki.c2.com/?CapitalizingGod has the following story, but i don't know if it is correct.
We may remember that ancient teletypes, printers, and terminals would bang out their output in all upper case. When this technology was first being standardized, the researchers did usability studies and found that people read lowercase text more easily than uppercase. But management decided to use uppercase instead. The reason? If only lowercase was shown, then there would be no means for CapitalizingGod.
anyway, by the time people started putting screens and pixels on computers in the 01960s to 01980s, the uppercase-only tradition was very well established.
probably to make sensible use of limited display abilities of early hardware, fitting characters in 8x8 boxes and making it readable on blurry fat beam glowing green screens quickly becomes a problem with lowercase (size, readability, ascenders, descenders, baseline), whereas uppercase is comparatively easy.