There’s something rebellious about the beta male; he challenges the social order rather than succumbing to it. The beta male doesn’t buy in to the basest stereotypes about male behavior, and that’s hugely sexy.
A beta male is often introverted, intelligent, and introspective. Though he may have been branded a "nerd" growing up, the adult beta is frequently a thoughtful, capable, and fascinating individual whom many women find appealing.
Alpha males, especially those of lesser intelligence and abilities, tend to feel threatened by beta males, who are increasingly preferred by women in modern times.
Plenty of insecure, pushy, loud guys are betas: most frat boys (especially the toady), most Ivy Leaguers, most football players, most Tea Partiers, most suburbanites, most rednecks, etc. Since they 1) don't have followers they aren't alphas and 2) don't have much of a mind of their own aren't gammas.