The main reason is that people don't understand science, but they will still appeal to it as if it were the law of the universe. It's an appeal to authority: Much like people used to say "X is true because God said it", it is now "X is true because Science proved it". Science is a continual work in progress towards an objective understanding of the universe. But it's so easy to see a news story saying: "A new study shows that global warming will melt the ice caps before 2012! More at 11". People have been lied to before and don't trust these studies, because they may have faulty conclusions. We need to look at the data and make our own conclusions, because we may all have different perspectives on what caused a result.
This is why I feel so uncertain about almost everything. How can one go through life confident about anything if they must not only research everything for themselves, but also overcome their own biases? What makes somebody qualified to evaluate or research anyway?
I feel like after over two decades of living, I don't know a damn thing, and I don't mean that in a "I'm so young and naive" sense either.
I disagree that it is simply an appeal to authority. I think this is more accurate: "X is likely true not just because the current scientific consensus says it is, but because we as humanity literally can not know better".
Relevant reading: http://en.wikipedia.org/wiki/How_to_Lie_with_Statistics