I think there's two other interrelated things that contribute to this.
"Big Science" these days, especially in healthcare, often involves long-term, expensive studies which take years to perform. People who commit to this mode of science make both a commitment to the field, but often to the hypothesis being tested.
To get the study funded requires basically betting your career on the validity or at least the likelihood of the validity of the hypothesis.
So, if I've bought into the hypothesis that dietary cholesterol influences serum cholesterol and it takes 10 years to design, fund and implement the study involved in it when the results turn out negative, what of my career? I've invested a good chunk of it basically being wrong.
And I think a fair amount of the people involved in these big theories aren't just scientifically interested in them, they are invested in them in terms of scientific reputation since they kind of have to be to get them funded. They often become advocates for the theory before it's proven, and if it isn't sustained by the study there's the risk of looking foolish because you were wrong.
So between personal reputations and career commitment and the size of the science involved, people have a lot of personal stake in seeing their hypothesis validated.