Africa is a cradle of humanity in a sense that apes evolved into humans in Africa, then those first humans stayed exclusively in Africa for some time, then some of them moved to other places and evolved in different ways there according to different environments (like lots of melanin is beneficial in hot sunny environment but is bad in colder places with less sunlight etc.). You can read more
here.
They are citing University of Pennsylvania genetic scientists which are the source. Also if Africa is where first humans came from, it just makes logical sense for Africans to have the most genetic variety among everyone.
MKX to MK11 Kano is actually a good transition for a change. MKX to MK11 Cassie is ok, which is a rare exception for an MK11 female character.
Kotal is probably the best transition they did, Jacqui is hands down the worst of the worst. In general female characters got it much worse, also some males were definitely butchered as well, such as Johnny Cage.