At sunset, sunlight must travel through a thicker layer of atmosphere to reach our eyes.

When the sun goes down, it sits low on the horizon. Because of this sharp angle, the sunlight has to travel a much longer path through the air to reach you. It is like swimming through a thick pool. By the time the light gets to your eyes, almost all the short blue waves have been scattered away and lost. Only the long, tough red and orange waves can make it through the thick air.