Because of ignorance and fear.
Most people still believe everything they see or hear on TV, and hardly ever do any research of their own. After all, they wouldn't lie to us on television, would they? Besides, they've already done all the thinking and analyzed the news for us, and their conclusions sound good enough. There couldn't be another point of view, could there?
And then there's the fear of being wrong, or of considering the truth to be too horrible to be... well, true. These intellectual "defeats" don't fit in the comfortable pattern of life that most people have purchased and conformed to; the mere thought of it is unbearable.
Therefore, since the message is intolerable, the messenger must be attacked, ridiculed and -if possible- corrected, so the system is kept safe and can keep working properly.
Being open minded means having to be willing to confront one's own knowledge and ideas to the fact that they may be wrong or incomplete.