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My working theory on this is because of how HTML is rendered in browsers. Back in ye olden days, when one could write a website's HTML entirely in Notepad, whitespaces characters were used for formatting as well as code indentation. Browser engines of that era would treat a single whitespace as a formatting space, and so, you would visually see that in the rendered output as a space between words, after punctuation, etc. But if there were two or more spaces, the browser engine would only render a single space, and this was because one would use those extra spaces (and TABs) to indent HTML tags for readability.

If you wanted more than one space to be rendered, you had to throw in a non-breaking space entity,   (char code 0xA0 in the extended ASCII set, U+00AO in Unicode, and ALT+0160 in Windows' numeric keypad). Hence why back in the days, if you looked at the page source of a given site, you'd sometimes see long lines of       throughout the code; usually this was used as a hack to force a specific formatting, before people better learned CSS to do the same task.

I think that because of this really tiny nuance back then, children and young adults who grew up in that era got used to seeing only a single space after punctuation, even if there were two or more, and they adopted this in their writing styles. By the time professional writing organizations figured this out, it was effectively too late to change this now-ingrained behavior, so there's been a push to get people to just stick to using a single space after punctuation, and it'll be up to rendering engines to add any needed graphical enhancements to make distinguishing between sentences easier.

That said, as someone who grew up doing two spaces after punctuation, that's what I continue to to do, and will continue to do, until those two spaces are pried from my cold, dead (or undead) hands. And yes, this entire post has two spaces between each sentence, though it may not be possible to observe it, as I don't know if Reddit strips extra spaces out or not (since in its Markdown mode, two spaces at the end of a paragraph is regarded as a break between paragraphs). If someone wants to check the underlying code in an inspector, that'll reveal if it preserved my two-spaces between sentences, but the rendered output will likely carry only a single space (this can vary between browsers and platforms).