Hayden is probably more expensive to book, though, so it was a weird choice for KH3D where she only had one line as well. I guess they wanted to share the love?
I feel like not only that, but Hayden's schedule is really the deciding factor. We all know that Alyson can do Xion perfectly, IDK how I would feel about her taking over a large chunk of Kairi's dialogue, but she fits as Xion. However, with Hayden, IDK how I'd feel with the opposite, her taking a large portion of Xion's dialogue, but she fits Kairi.
I do think KH3 will see the return of Hayden, I can't imagine them having her miss out on KH3, where Kairi most definitely will speak A LOT O_O