The answer is because alfa males spreads smells that attracts female. At least in rodents this is proven by a study with a nice design. They first exposed female mice to soiled bedding from male mice as a source of pheromones. They proofed that a 7-day exposure of female mice to soiled male bedding increased the production and survival of new neurons in the hippocampus. The authors also exposed female mice to the bedding of castrated males. Castration removes pheromones from the urine by eliminating circulating testosterone. Bedding from castrated males did not stimulate neurogenesis. This pheromone-induced neurogenesis appears to ultimately determine female mating preference for dominant males. The preference of female mice for dominant males is explained by pheromone-induced neurogenesis in the olfactory system and hippocampus.
From the articles:
Nature Neuroscience – 10, 938 – 940 (2007)
Alpha males win again, by Derek P DiRocco & Zhengui Xia
Nature Neuroscience – 10, 1003 – 1011 (2007)
Male pheromone–stimulated neurogenesis in the adult female brain: possible role in mating behavior. By Gloria K Mak, Emeka K Enwere, Christopher Gregg, Tomi Pakarainen, Matti Poutanen, Ilpo Huhtaniemi & Samuel Weiss