Why?

The answer likely involves multiple factors.

First, the selection pressure for lighter skin, while real, may have been weaker than assumed — because the hunter-gatherer diet of this period was rich in vitamin D from animal sources, particularly fish and organ meats, reducing the dependence on skin synthesis.

Second, the time required for natural selection to shift the frequency of pigmentation variants across an entire population is not trivial — it requires many generations of differential survival and reproduction, and even relatively strong selection pressure takes thousands of years to substantially change allele frequencies.

Third — and this is a crucial point from the population genetics evidence — the actual genetic change in European skin pigmentation was not simply a gradual evolution of the people who were already there.