Is there some explanation from an evolutionary standpoint of why generally speaking black people have kinky hair where as most other “races” do not? It just seems odd to me the striking difference as if some environments strongly favored the tightly curled and some did not, but it doesn’t seem like that would be enough of an evolutionary pressure to change something as seemingly trivial as hair.
Although we can’t know for sure, it’s likely that the “kinky” hair of sub-Saharan African populations is the basal phenotype, and that straighter hair of other populations is a more recent mutation. Also, note that the hair texture of the Khoisan people is a bit different than what most people think of when they say “kinky” hair, and that might even be closer to the basal condition-- it’s even more tightly coiled than that of other African populations and is sometimes referred to as “peppercorn” hair.
Generally speaking, it’s far easier to change a “trivial” attribute than a major one. Skin coloring, size and shape of ear, nose, and lips, ability to manufacture the lactase enzyme as an adult are all the kind of small changes and variations that can form with small alterations to a gene or gene set.
It’s the really large changes that would be surprising over so short a time, and in fact those are exactly what we don’t see. Today’s Homo sapiens are almost exactly like the Homo sapiens of 30, 50, and 90 thousand years ago except for the trivialities like hair and skin and eye coloring. Those have relatively minor effects on survival (although light skin may help in colder climates) and so can go through wider variations without affecting anything serious.
As for the “trivial” change of hair texture, it could very well have been sexual selection-- somewhere along the line, one group of humans found straighter hair to be more sexy. Or, it could have been simple genetic drift or a founder effect. It needn’t have been selected for in terms of survival value or even reproductive value. It might just have been an accident without any evolutionary angle to it. Traits get weeded out if they reduce reproductive success, but that doesn’t mean every trait that remains increases reproductive success. Sometimes stuff just happens. 
If you look at all of the “oldest” populations of humans today, though, they all have tightly curled hair. You see this in Africa, and the more ancient non-Africans (like the Andaman Islanders). That is why it’s probably the basal condition.
Also keep in mind that humans have every degree of “curliness” between the tightly curled peppercorn hair of the Khoisan peoples and the very straight hair seen in many Easy Asian populations.
What makes you think there’s an evolutionary explanation? Mutations are random, and they’re passed on, unless they kill you before you procreate.