There are different reasons.
There is some association between lighter and darker skin colour and wealth. More exposure to sun = darker skin = poor, farmer, worker. Lighter skin = wealthy, doesn't have to work outside.
Lighter skinned people are dominant everywhere. Indian castes appear lighter as you move from Dalit to Brahmin. The Tutsi are lighter than the Hutu. In the Dominican Republic, in which at least 90% of people have discernible African ancestry, they still prefer media featuring people with the lightest skin.
This has something due to colonization through European powers. For example the Tutsi and Hutu mixed and the skin colour didn't matter. It was a social caste system. A Hutu could gain Tutsi status through marriage or economic success. Then the Belgian arrived and started to divide Hutu and Tutsi after physical appearance. A Hutu with a lighter skin colour was considered as a Tutsi by they Belgian. Same goes for India where you have the Indo-Aryan immigration theory and the fact that British colonists treated Indians with lighter skin better than Indians with darker skin.
"If you're light, then you're alright" (PDF warning):
Much of the literature on colonialism and slavery focuses on the domination of African and indigenous peoples by Europeans, and skin color is often discussed in this literature in terms of the creation of racial hierarchies. Light skin is associated with Europeans and is assigned a higher status than darker skin, which is associated with Africans or indigenous people, and is assigned a lower status. These colonial value systems are forced on the colonized and often internalized by them.
And there is western media influence. Hollywood films are popular and commercials with Caucasian actors are not uncommon in countries with a non-white population..