All 3 languages have a bit more letters that English. Some of those language's letters are marked with inflections.
Plus the first computers used only Upper Case Letters which were designed similar to how Ancient Latin was carve letters in stone. So it was far easier to design a printers, storage, punch cards for when you only care about 26 letters and 10 numbers and a few punctuation marks.
The “first computers” (which can actually mean a lot of different things depending on what you’re defining as a computer) didn’t output text at all.
Even in the digital era, they output binary rather than text. In the 50s it would have been machine code in and machine code out.
Punch cards would have been binary, Initially machine code but later text encodings in binary. Given machines back then weren’t fixed with 8-bit bytes, it meant you have have larger or smaller character sets.
There were plenty of Japanese, Chinese and Russian computers using non-Latin characters. There were also plenty of European computers that supported native characters outside of the standard 26 English letters too. That’s why character encodings have been such a nightmare to work with prior to Unicodes adoption (and frankly, Unicode creates a new set of problems, but that’s a different topic entirely).
Just because you haven’t had to work with non-Latin, or even non-American, encodings doesn’t mean that all machines were English-centric.
Most of the lowercase Cyrillic letters are just smaller version of the uppercase ones. And neither the German nor the Russian alphabet is larger to any extent that matters.
CJKV languages are a bit more subtle subject. Vietnamese nowadays uses a romance-inspired alphabet, where only the tone marks are slightly difficult to typeset. Japanese and Korean could have gotten rid of Kanji/Ganja if they really wanted. But in any case printing technology for Chinese characters existed and was in wide use at the turn of the 20th century.
French and German are both completely intelligible when written in all caps with no diacritics (in French by simply omitting them, in German by replacing e.g. Ü with UE).
