You don' t think the Brits history as the first major power to not only outlaw slavery at a time it was common in the world and practiced by every race in the world is pretty amazing.
And it was not all talk, Great Britain went so far as to lose thousands of sailors attempting to end the slave trade for decades as it continued between the African tribal chiefs and Arab slavers.
Of all the nations on the planet including the US, no country in the history of the world did more to try to end the practice of slavery than England.
The sad reality is even with the efforts of England and subsequently the US to end slavery, it continues. So why do you think the focus is on only slavery in North America during a very short period of history relative to slavery as an institution and its ongoing presence today.
There are more people enslaved today than at any time in history. It would seem Markle would focus on that issue and helping those people today that are victims of human trafficking, if awareness of slavery was her mission.