Simple. Because they get most of their information there. Most people choose not to think, but rather to believe what they hear most. A big difference. This is also why every dictator hates to have uncontrolled media. If you control the media, you control the people, or at least what they think.
Because people feel secure agreeing with what is mainstream, with what everyone else Perceives as normal because if you think like how everyone else thinks that makes you "normal". What I think people fail to see is that being "normal" and like everyone else is not always correct, or ok.
Because humans are social creatures and we succumb to peer pressure because we biologically do not like the feeling of awkwardness or being alone. This is a 'bandwagon' effect that can be applied to religious, political and social causes.