It's not that simple. The formats differ structurally. Org-mode is a proper tree - that's its fundamental basis. A tree where each branch can hold k/v pairs of metadata. Markdown is rather a flat sequence of blocks with level markers sprinkled in. Neither of that (proper tree or records) fundamentally supported in Markdown within an agreed spec - pandoc attach attributes on its own way, kramdown does it differently, etc. That's the most difficult part - there's no agreement of what Markdown is - each tool treats it differently. While Org doesn't have a formal spec either, there's no mess, because "the spec" is in the Elisp code it ships with. Even if there's fragmentation (Org-Roam, Denote, howm, vulpea), it is all contained within emacs-land.

If you want to use markdown for Agenda and calendaring in Emacs you'd have to invent a different subset of markdown, most likely poorly supported anywhere else. And if you get there, then it begets a question - why? If we already have something far better and it's called Org-mode?