There's only one answer to this question...
Scientifically, there is no such thing as race. One person could have two "black" looking parents, and one "white" grandparent, and look fairly white, while another person could have two "white" looking parents, one "black" grandparent, and look primarily black. It happens.
The assumption that different "races" have different physical or mental attributes from one another, on the whole, is called "scientific racism," and it was fully disproved in the 1800s.
It is true that this for some reason this idea has not seemed to sink into western society....
Another example is in running. For the past ~20 years, "black" looking people have won a huge percentage of high-profile running races (the Olympics, and what not), and so people recently have been suggesting that they are somehow physically better equipped to perform.
This is not true, however. It's entirely cultural. They win because, in the case of running, there are entire villages in Kenya and Ethiopia effectively entirely dedicated to producing such athletes. Their culture is streamlined to that one goal, whereas western society doesn't have that kind of focus, or even treat it as "normal," so of course it's hard to compete.
Also, these things come in waves. In the 30s and 40s, it was the Scandinavians who were dominating athletics, and people were making the exact same, incorrect assumptions that they were "athletically superior." Before that, in the 1900's and 10's, it was the British. Before that, it was another group, and etc., so forth.
The point is, "race" has nothing to do with it, and even if the majority of good bass players today WERE "black," it would be because they were enculturated with the idea that music is important, and they should practice.
Thanks for reading my rant. If you don't believe any of the facts I gave, message me and I can give you citations.