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.