Answer:
social: white people believed themselves to be much more civilised and humane than coloured people, so they had no issues using and abusing any coloured people, and this allowed them to use slaves without any judgement.
political: those who participated in the slave trade began to have a flouring economy which gave them power over countries witch more poverty. additionally, the slave trade made certain countries integral in the production of goods which gave them more power on the world stage.
economical reasons: slave trade was very profitable for those selling the slaves as well as those using the slaves for free labour. it benefited the economy of society as a whole by allowing for mass production and therefore mass amount of profit.