Bottom line is that it's easier to be white (most of the time). White people suffer less discrimination.
And in old slave countries and those countries' communities, it desirable to have whiter skin because then you are deemed to be of a higher class/caste/status rather than having worked in the field or being a slave descendant.
My family are from Jamaica and my grandfather's family are from Cuba therefore we're lighter skinned (and we have less black features) than my grandmother's siblings' families (dear God all the apostrophes! I'm getting very confused) and we receive more respect from the neighbours and the community. It's stupid, but that's the way it is.
Lighter skin is seen as being more beautiful, if you look a beauty pageant winners from African, Arab and Caribbean etc countries, the woman chosen to represent the country is usually lighter skinned.