Because it’s not centered around the Skywalker family, and I wish Lucasfilm would realize that. It’s not the Skywalker Saga, it’s just the Saga.
“The Skywalker Saga” is retroactive marketing bullcrap that they used to try and promote the final film. The sequel trilogy is not about the Skywalker family, nor should it be. No, Rey shouldn’t be a blood-related Skywalker because that would be reductive and stupid. Even more reductive and stupid than making her a Palpatine.