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.

Relevant reading: http://en.wikipedia.org/wiki/How_to_Lie_with_Statistics