Why did slavery shift from indians to africans?

They were less susceptible to infectious diseases than the Native Americans having never been exposed to them before.

The  pan-african movement  was an "emotional, cultural, psychological and ideological  movement  that began among the  african  dispora in the western hemisphere, for a closer  purpose, so that  african  people could feel secure, attain political, economic as well as psychological power visa vis other races or world regions" .