I think I can answer this one authoritively. It was a major part of my working career in high-end publications (e.g., HP user manuals for LaserJet printers, etc.).
I learned to type on a manual typewriter, and the rule was two spaces between sentences.
However, with the advent of "desktop publishing" because the software determines where lines break, it switched to one space between sentences because the software would only recognize one space to throw out at the end of a sentence when a sentence ended at the end of a line. If there were two spaces after the period, the next line would start with a space.
Also, in the case of justified type (even margin on both sides) where extra space is added between words and sentences to make the right margin even, the two spaces would make a noticeably larger space between sentences if there were two spaces.
~Binadub