Because it's great but it's quite unlike the movie. It's much more Flash Gordony than ANH turned out to be. It relates much more to our universe, our world in a way... "a billion years in amaking" suggests it's about the future and stuff. While in the end it's a fairy tale.
And yet, it does the trick, it sells the film. Especially knowing that the effects shots in there were the only ones finished at the time. Movie magic, that's what it is.