I'm not sure I'd qualify TVML as another mark up language. It looks like an XML standard that adheres to a particular XML schema. Asking, "Another XML schema?" would sound like a silly question because that's the whole point of XML.

As for why not HTML? I think there's a lot of really good reasons why HTML would be a poor choice. If you want to restrict Apple TV apps and channels to rely on Apple approved idioms, HTML would be a terrible choice. Apple isn't as interested in creating an open TV browser as much as it's interested in creating a unique and positive TV experience. They can shape that with their own standard.

Much more specific and limited semantics. I would not expect that this is rendered with a web engine.

because a TV screen != Hypertext Document.

Documents aren't applications.

I really would have liked that XHTML had won.