There is a good answer to that as well.
Aside from any socio-econo-political reasons that may or may not be valid.

The body of a black person is more dense than a white person, so they tend to sit lower in the water and it's therefore harder to swim fast.

Conversely a black person has relatively longer tendons compared to muscle in their legs, allowing them to run faster, hence the dominance of black runners in sprint events.

Both these facts are at least second hand, and should be checked before being quoted on QI. A pub quiz you'd probably get away with it.
(Spidge New sig needed, donations gratefully accepted., Wed 7 Apr 2010, 12:36, 3 replies)

^^^This^^^
I have heard this too - but have no evidence to support it.
( Snappyuk gets BIGGER with free sample., Wed 7 Apr 2010, 13:09, closed)

African blacks have more genetic and phenotypic diversity than the rest of the human race put together
So while it may be true that the fastest sprinters are black, the best long distance runners, etc. it is probably also true that the worst sprinters and long distance runners are black (but harder to measure). For any given human parameter, the chances are that the distribution curve is somewhat flatter for black people than it is for other ethnic groups. Mainly because there are many different ethnic groups within the heading "black people", united only by their skin colour.

So the bodies of some black people may be more dense than the average white person. Other black people will be less dense. Some may have relatively long tendons; others will have relatively shorter ones.

Of such generalisations are prejudices made...
( shinyshinyscalp less a man, more a way of life, Wed 7 Apr 2010, 14:21, closed)

Of such generalisations are prejudices made?
Believing that black people have longer tendons or are more dense is never going to lead to prejudice unless you're seriously unbalanced.

No more so than believing they have higher genetic diversity than white people, anyway.

Generalisations are a good thing, they allow a better starting point for understanding things than having to start from 'new thing, what is it?'.

Also, assuming you're right (and you used bigger words than me, so could be), we're still lacking an explanation to why black people are notably absent from swimming events.
(Spidge New sig needed, donations gratefully accepted., Wed 7 Apr 2010, 15:01, closed)

Wrongness
"The body of a black person is more dense than a white person, so they tend to sit lower in the water and it's therefore harder to swim fast."

If you are lower in the water then you can swim faster more easily, hence the rules about how far you can travel under water when pushing off from the pool end.
(Falco, Wed 7 Apr 2010, 16:13, closed)