The British Empire fought like the dickens to gain for itself that Asiento.
They WANTED to be the slave trading monopoly. They succeeded in 1713.
Here is Britain several hundred years later, receiving it good and hard.
That said, you are correct it is strange that both Spain and Portugal, as well as do not forget France and the Netherlands. All have stained hands.
It is very nice to see the U.S. not be blamed first for a change though. We did not exist until 1776.