An important theme that adds urgency to the show and explains why it makes sense to recount this particular time period is the Wall. It has stood for over a thousand years but now the Lich King is coming, the wildlings are attacking it. If it falls this is a big event that changes everything for everyone, the dance for the throne by all the important houses will no longer be the same.

If you make a prequel you can't really topple that. If you make a sequel in normal fashion you need to make it even bigger, die hard 1 a building, die hard 2 a plane, die hard 3 many buildings, in GoT that would be something like "there's a Demon Lord that's been slumbering for 5000 years that someone just awakened".