I think the main reason is that there is not a lot of demand for another standard. you already have HTML and XML. But almost no one use XML to render documents

I think the main reason is that there is not a lot of demand for another standard. you already have HTML and XML. But almost no one use XML to render documents in web browsers and it will likely go away at some point and HTML will stay the only standard. In this context, it seems unlikely to add Markdown to save a few bytes.