Because our assessments of the outcomes of different phenotypes have been hopelessly polluted by 16th - 19th theories, often based on the Bible, that have caused people to unleash untold and unrelenting violence on people with
naturally servile phenotypes for dozens of generations.
That being said, slavery and colonialism are clearly dysgenic. Slavery kills the brave, the strong, the honest, the compassionate, those who can see the bigger picture, those with self-respect, those who love their parents, those who love their children, those who will not be raped, those who will not be enslaved. It selects for the apple-polisher, the scammer, those who are best at concealment, the selfish, the ruthless, the wily, the drone, those who can bear endless misfortunes, those who can lift heavy weights.
"Readily trained" isn't synonymous with "highly intelligent," although readily trained is the goal of breeding. To be "readily trained" is to be obedient; ready to take the master's priorities as your own.
Even with full reparations and the lack of further obligation that would mean for the US, black people will be in the position of trying to silence that part of our brains that has been bred to prioritize the problems and the presence of white people. Luckily it was only 400 years of damage.