Browsers implement well-defined web standards, which Markdown unfortunately isn't. The original author, John Gruber (of daringfireball.net), hasn't been too supportive of having a single, fixed Markdown standard, and this has resulted in spinoffs with various syntaxes and features:

- GitHub-flavoured Markdown

- CommonMark

- etc...

On top of that, there isn't even some meta-standard which would allow hashbang-style preambles/doctype declarations to determine which version of Markdown is being served.

So, between the fact that there's no fixed de-facto standard and no de jure standard syntax or feature set recognized by any of the major standards bodies (IETF being one of these), it would be impractical for browser vendors to implement support for Markdown.

Finally, the standards that browsers implement take time both while being defined, as well as while being implemented; these are things that will be seen by billions of people worldwide, and there's extremely little room for error, so revisions are always necessary at every stage in development. For example, work on HTML5 can be traced back as far as 2004, meaning that from inception to release, development took about a decade.