So there are two parts to evolution. There is the occurrence of random differences due to genetic mutation and then the selection of those differences, i.e. natural selection.
Here you've got the random difference in that some guys have longer foreskins than others but what about the selection?
For a characteristic to be selected for it has to mean that either the individual who first has the characteristic, or those who inherit it, must eventually outbreed those without the characteristic so the characteristic becomes the new normal for the species.
So, as an example, if a mutation occurred that caused some individuals to be more efficient in their use of food energy than others then maybe to start with nothing much would change but as soon as their was a severe shortage of food, more individuals with that characteristic would survive and breed than those who didn't have it as those without would be more likely to starve.
Likewise, if a characteristic makes the male more attractive to females so he has more offspring the characteristic will spread.
If a characteristic causes a problem for the individual with it then it is likely to die out. if the characteristic confers no particular advantage or disadvantage then it may persist within the population but it won't become dominant. Of course, it may be tested many years and generations later so, for example, immunity to a disease that is not common isn't really being tested. If that disease were to become common, though, suddenly those with the immunity survive and the others die causing the rebuilt population afterwards to share this characteristic.
So, after all that, no particular foreskin length has been selected for because it doesn't make much difference to survival/breeding rates.
