But "why" ?

Well, consider a polymorphic proto-human population in which you find individuals that can grow nails and others that cannot. Female individuals with growing nails ability would have a selective advantage (in terms of reproduction) since they would be able to wear nail varnish without fearing a broken nail. Thus, these [GrowNail +/+] females would be preferred for secretary positions and would therefore enhance their mating chances.
Conversely, female who can't grow nails would take only low-rated jobs such as stone-carving or fire research. It's a well known fact that scientists of both sex have never been considered attractive.
Therefore, the [grownail +] allele of the "GrowNail" gene would rapidly spread in the population.

That's basic Darwinism.
I'm glad if I could help.