i dunno what the dominance/recessiveness of the red hair or blonde hair allele. In any case where I am assuming that red hair and blonde hair are recessive alleles, it is basically because the dominant allele (we could assume for argument sake is brown or black) would increase in frequency...
You have to understand that and increase in frequency in a population for the dominant allele decreases the frequency of the recessive allele...I dont know if the recessive allele can COMPLETELY be wiped out, i think it can just decrease over time...AA x Aa = 1AA:2Aa:1aa, so the probability is 1/4 but it could still happen that the blonde hair and red hair exist