Based on the ideas of Mark Twain, John Stuart Mill, and Plato, why is the force of the "opinion of the many" so powerful on individuals?

According to Twain's "Corn-Pone Opinions," what drives people to engage in group think is their preoccupation with their capital--both literal and social.  To stand out from the crowd with independent thought risks rejection, a state that Maslow's hierarchy of needs suggests violates our human desires for safety, love, and belonging. Ever the pragmatist, Twain recognized that people's opinions were formed by "calculation and intention." 
John Stuart Mill believed that we are all sovereign to ourselves and entitled to freedom of thought, expression, and unification with others (as long as that unification doesn't harm anyone who is a part of it).  Mill felt that there must be mechanisms to ensure that the majority doesn't oppress the minority. Mill understood that being "right" is relative and saw no problem with free thinking and discussion and embracing multiple viewpoints to move humankind closer to life's truths.
Plato was a proponent of social contracts, and his allegory of the cave suggests that it is justifiable for leaders to deceive their people because the people ultimately receive the benefits of the community.  Plato's and Mill's philosophies are fundamentally incompatible in that Plato advocated putting a minority in power and stratifying (and, arguably, infantilizing) the majority. Mill had faith in people to be rational and make their own decisions; if there were violators who hurt others, they would be censured.  
The force of the "opinion of the many" is so powerful nowadays in large part because of social media and the consumerist model that governs so many lives.  Twain's satire seems remarkably on target as people crave "likes" and "retweets" as evidence of their social capital.  Many people collect Facebook friends and LinkedIn contacts as affirmations of personal worth and professional success. Because so many businesses also rely on social media for marketing purposes, their "brand's" viability is dictated by its approval and popularity as measured by not only sales but its positioning in public media. 
To bring this discussion back to Maslow, it is arguable that the powerful force of the opinion of the many that governs so many people's lives prevents them from achieving self-transcendence, the tiniest segment at the top of his pyramid, revised in the late 1960s and explained by Maslow as "the very highest and most inclusive or holistic levels of human consciousness, behaving and relating, as ends rather than means, to oneself, to significant others, to human beings in general, to other species, to nature, and to the cosmos."