Fear of being different or exposing themselves to ridicule by others that they seek approval from. But also cultural conditioning. If from a young age most people are fed certain ideas, values, morals, etc.. and these constructs are taken to be real then it becomes absurd to think any other way.. just like albooboo said above. Also the fact that there may be limited positions, views or outcomes to the argument or discussion in the first place again due to cultural conditioning.
But the want for acceptance plays a big role. The want for approval or to fit in... in a society and culture that plays to much on monetary wealth, appearance, deluded concepts of success... the pressure to perform and fit in is so great that subconsciously people will naturally gravitate towards a more general and widely accepted view point on any matter. And the stakes get higher according to the weight of the issue discussed.
This culture is built on fear. And seeking comforts that will present a certain appearance that will be awarded with whatever form of praise is fitting. Social, political, economic, religious and scientific paradigms usually have to run their course to their respected inevitable failures before a new way of living or viewpoint can be comfortably adopted by the masses.
The majority of western peoples minds are weak and conditioned.. you are delicately fed what you are intended to attach to and subconsciously help propagate. Mankinds greed, ignorance and fear will be it's downfall.
Jiddu Krishnamurti's famous talk which some of you may know from the beginning of the first Zeitgeist film still stands the test of time:
"We were saying, how very important it is, to bring about in the human mind the radical revolution. The crisis, is a crisis in consciousness. A crisis that cannot anymore accept the old norms, the old patterns, the ancient traditions. And, considering what the world is now, with all the misery, conflict, destructive brutality, aggression and so on... man, is still as he was, is still brutal, violent, aggressive, acquisitive, competitive... and he has built a society along these lines."
And that concludes my wrathful and borderline misanthropic input for the day.