Why was the Iberian Peninsula first? Because it was controlled by Muslims for more than eight centuries which ended completely only in 1492. The Muslims brought slavery into a world where there was no slavery after the Roman Empire fell.
In fact, they invaded from Africa for much of those 800 years to raid villages and take slaves. It was only when the native populations grew strong enough to resist those raids that the venue moved to the high seas, particularly the Mediterranean, a practice when ended only in the early 19th century when the ships belonged to countries which either (a)paid them regular tribute or (b)would open a can of whoop-@$$ on them as we did on the Shores of Tripoli, 1805.
So, the Iberian powers, in particular, felt it only natural that they be able to return the same favors as had been visited upon them in previous eight centuries. The fact that most of the African peoples pressed into slavery were mostly not Muslim and largely collected and traded by Muslims were lost on them.
Not so different than the fact that all of the white folks targeted by today's Black folks never owned slaves.
When the Portuguese began to explore the African coast, the pope gave them approval to enslave the natives--but only those who were not Christian. Religion was what determined if you were fair game, not skin color.