Plain html has two things going against it.
First, it doesn't have any provisions for code reuse. So, if you have multiple pages that use the same header, same footer, or same navigation menu, your options are either to copy-paste it (gross), or to build the final html out of smaller pieces, at which point you've reinvented either a static site generator or a web server.
Second, if you write long stretches of text, the html markup can get in the way, as opposed to unobtrusiveness of something like markdown.
