Why is it that the mainstream is comprised of large numbers of people with little political / philosophical diversity (e.g., roughly half of Americans identify as Republican or Democract), but when you get into the tails, there are small numbers of people with tremendous political / philosophical diversity (e.g., at a meeting of any fringe group you're likely to be inundated by a tide of nuance, such as arguments about the merits of anarcho-syndicalism vs Trotskyist Bolshevism, or whether #libertarianism is compatiable with anarcho-capitalism)?

In other words, why do labels increase as population decreases, moving away from the mainstream into the political fringe?