A good reason why we say brown dolls for all kids is because we really believe that it’s important that ALL kids engage in play with diverse toys, which means all the colours of the rainbow included.
For the longest time, those options weren’t there, things are changing, we’re happy to be a part of that change.
Different Shades 
Boys and Girls
Essentially when playing with dolls, children see a friend, an equal, someone to care for, nurture, teach, talk to, protect, and treat as a human. If you only learn to practice these skills only on one kind of doll, and you add media beauty standards to the mix which has an abundance of westernized everything- you will find the result is less regard for anything other than what they interact with. The message inherently is that everyone else, and everything else is less important, less human, less deserving.
Even as adults if you see anything constantly, in volume and widely available you start to perceive that to be the accepted standard or the aspiration. So what message does seeing 20 different versions of white dolls versus 2 black dolls in a shop give? Then you go home and watch TV and all the characters that go about their day, have dreams and love their wonderful lives are all exactly the same, look nothing like you (or they do which is great if you fall into that group, it’s self-validating isn’t it?).
Being aware of diversity reminds us that we are all one thing, human, although we come in different shapes, colours and sizes.
This is why African dolls, black dolls are an important aspect of play- if you choose to have dolls for your little one that is, because diversity, inclusion is important for all of us.