The odds of finding like-minded people are much lower with a smaller pool of individuals

Lifestyles or interests that deviate from the mainstream need critical mass to survive; they atrophy in smaller communities not because those communities are more repressive, but rather because the odds of finding like-minded people are much lower with a smaller pool of individuals. If one-tenth of one percent of the population are passionately interested in, say, beetle collecting or improv theater, there might only be a dozen such individuals in a midsized town. But in a big city there might be thousands. As Fischer noted, that clustering creates a positive feedback loop, as the more unconventional residents of the suburbs or rural areas migrate to the city in search of fellow travelers. “The theory…..explains the ‘evil’ and ‘good’ of cities simultaneously,” Fischer wrote. “Criminal unconventionality and innovative (e.g. artistic) unconventionality are both nourished by vibrant subcultures.” Poetry collectives and street gangs might seem miles apart on the surface, but they each depend on the city’s capacity for nurturing subcultures. 

From “Where Good Ideas Come From: The Natural History of Innovation” by Steven Johnson