How did marketing to subcultures come about?

In the advertising and marketing world, newer, lesser established, smaller agencies continue to win more business and grow, while most large, established agency holding companies struggle to maintain profit margins and accounts. What could possibly be the cause?
Audiences are smaller, more fractionalized, more difficult to define, and easily distracted with choices. Marketing and media professionals, are versed with the shift of consumer culture from a passive audience of unity, to an interactive audience of plurality. A Latin phrase describes today's society;"E Pluribus Unum" to "E Una Pluribu," (from "from many come one" to "from one come many"). America have become a culture of plurality. And so they have all those sub-cultures to market to, which could make market research a bit tedious, and marketing a bit difficult to make it be attractive to the audience.