The point of markdown is also to use common characters available on the vast majority of keyboards to derive it’s syntax from, making it ‘easy’ to do formatting and such. I understand that your keyboard supports it easily enough, but most people’s don’t…
So one reason for not having it in the spec as a list marker is because people who can’t easily input the bullet (which is a lot of people at least right now) might be presented a situation where the markdown document they have received uses bullets. So they’d either have to change all the bullets to what they can easily use, or difficultly input bullets if they want to edit it, making it inconveniant, hard and therefore against the philosophy of markdown for them (which = most people). They’d need to do that because different list markers indicate a new list, so they can’t just mix your bullet with their preferred list marker, be it -, + or *, between different list items.
I thought the same with emacs and grid tables. Where emacs relates to your keyboard, and grid tables relate to the bullet character.
What’s a 21st century keyboard?