[Little Ray]

In 1493 Pope Alexander VI split the new world between Spain and Portugal. It allocated territory as between Spain and Portugal, excepting only those areas already ruled by a Christian monarch or power. The lands west of the line of dermarcation were given to Spain, and to the east, to Portugal. Portugal's lot eventually included Africa and Brazil.
The Portuguese were slave traders because their part included Africa, which had plenty of tribes and nations willing to sell slaves. Most likely the Portuguese simply made the best of what their market offered.

[livius]

Yes, I think the Portuguese just didn't want any interruption to business. That said, they were always more difficult in dealing with Rome than the Spanish. The only time Spanish rulers were wildly disobedient was when they constantly ignored Rome's orders to knock it off on the Inquisition (around the time of Felipe II, when the Inquisition had become nothing but an obviously political hunt for Royal enemies). Rome disposes - but people do what they want.