I like HTML and will use it for my own projects, but I cannot send pure HTML to someone who's not a into tech, so to speak, and expect them to actually read it. It doesn't take much CSS to make a readable web page, and I actually kind of like the barebones no-CSS HTML style, but for many others, that style reads as 'this page is broken'. I guess I can write all my styles inline or in the header, but that's a big ask when I never do that normally.

Markdown though, especially if you're not using way too much of it and mostly using it sensibly, just to give your document some structure, can be read as plain text by pretty much everyone, and will be implicitly understood. Sure, they might not understand the exact different between a word with asterisks on either side and one with underscores on either side, but they'll get that you meant to emphasise that work. They'll also understand a list shown with asterisks, while <ul> and <li> tags will get too verbose for them and clutter the actual content (I don't really care, but I get why they do).