I think that each of the Straw Hats are represented by color. Here is my chart, and look at it and find out why Brooke will never join.
Luffy-Red
Zoro-Green
Nami-Orange
Usopp-Brown
Sanji-Yellow
Chopper-Pink
Robin-Black
Franky-Blue
Now Robin is already the black color. So why do we need two? :wassat: