The answer is natural selection.
This while concept is that the ones with certain features that are well adapted to the environment have a greater chance to survive, reproduce and pass on their features to the next generation, comparing to those that are less adapted.
So originally there's both peacocks of with albinism and those don't.
Because albinism peacocks can be caught by predators easier and cannot attract mates, the number of then would decrease and they couldn't reproduce to pass on their genes.
Therefore, the ones without the albinism can survive easier and thus the number of them would be larger.
This is natural selection.
