User blog comment:Videogamep/The Next Straw Hat Craze/@comment-1876594-20140227174959/@comment-2226235-20140228172820

Why? It was pretty obvious that Shirahoshi wasn't going to join simply because she wouldn't fit on the Sunny. Why do you think Rebecca wouldn't join? She doesn't have any duties tying her to her home like Vivi or Shirahoshi since Viola would be next in line for the throne (after Doflamingo is defeated), not Rebecca.