With an all-white cast (or non-black cast) the conflict of the show is not race anymore, but social status, which isn't nearly as dramatic or effective. Also, the show takes place on a Carribean island, and if an all-white cast does the accent, they all just look like jackasses (trust me, I've been in one of those productions). After seeing an all-black version just recently, it became VERY clear that it just shouldn't be done with a multi-ethnic cast (unless it's high school or youth theatre). The power just isn't there.
Eeeeeeyyyyyyyyaaaaaaaannnnnddddd aaaaaaaiiiiiiiiyyyyyyaaaaaammmmmmmm teeeeeeeelllllliiiiiinnngg yyyyooooooouuuuuuuwwwaaaahh...