Leia (apparently) dies when Ren is stabbed,
become a force ghost
Ben becomes a ghost,
much later. Why not?
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We'll probably get something more explicit from the director soon, but based on the movie, it is because she is helping "kill" Kylo Ren and turn Ben back from the Dark Side.
As Maz says, what Leia is doing will kill her. From the Force skills we've seen her demonstrate, including the fact that we've seen she and Ren connect to each other earlier in the trilogy without much effort, it seems improbable that her saying "Ben" is as much as she could achieve before dying. Likely she is still with him; she doesn't move on to become a Force ghost until he does after his redemption with Rey, because Leia's connected to him and helping him.
There's not an explicit or canon-related explanation, but thematically, Leia had not "let go" of her interests in the material world -- in this case, ensuring that Ben has turned away from the dark side. To quote Yoda, "If once you start down the dark path, forever will it dominate your destiny." Once Ben was dead, his destiny -- and the chance he could turn back to the dark side -- was over.
This also mirrors the end of "Return of the Jedi," when Anakin's Force ghost doesn't appear next to Obi-Wan and Yoda until Vader's body is burned in his armor.