Yeah IDK. Wordpad is built around rich text, with all the weirdness and complexity that comes with it. I know for a fact that .rtf is absurdly complicated to work with, and I assume that .docx is similar.
I’m willing to bet that adding markdown to Notepad was a lot simpler than trying to make it work in Wordpad, especially since you’d probably still have to support rich text.
Syntax highlighting is definitely less complex than updating and rendering RTF and HTML.
There is configurable syntax highlighting in vscode.
Should an app like Notepad ever embed a WebView? (with e.g. tauri-apps/wry instead of CEF now FWIU)? Not even for a Markdown Preview feature IMHO.
Hence why I use .txt and not .rtf (After having multiple RTF files become corrupted)