A theory is an attempt to explain a phenomenon in the natural world and has not necessarily been proven. Throughout the years many theories have been proven and disproven. There are always going to be people with different points of views and theories of their own. Skepticism is always going to be there.
The beauty of scientific theories are that they remain theories until substantiated by facts. At some point, the theory, held together with sustainable facts, becomes fact itself, inasmuch as anyone today can agree on fact. But the reasons for dissension are not scientific; no one in Newton's day got upset with his Universal Formula for Gravitation; fact supplanted, supplied and expanded the explanations of celestial and terrestrial motion which conjecture had previously held.
Theories can be proven or disproven. Facts can only be established.
There are more failed scientific theories than successful ones, but each failure eventually leads to a deeper understanding and ultimately a stronger theory that can transform to fact.
Human egos are delicate things. As mentioned, the history of science is full of examples of young upstarts challenging and sometimes prevailing over the august theories of an older generation.
The theory of global warming can be demonstrated as fact; the world is heating up; the facts behind the warming cannot, at least currently. That its a function wholly of industrialization has not been proven, as it has not accounted for the natural cycles of global heating and cooling. Certainly industrialization may be accelerating the natural process.
Good scientists base conclusions on observable fact and change as new facts dictate; they do not base conclusions on emotions like so politicians pandering for a cause and for votes.
In addition to the reasons already stated, there is resistance to new scientific theories because the community of scientists rely upon each other to test eachothers theories, methods and even repeatedly tested experiments and their outcomes. The resistance to a new theory is especially high and scientists will be particularly skeptical if that new theory is purely "theoretical." Some scientific theories have developed based on thought experiments and abstract thinking - only indirectly based on recorded observations and past experiments. There are some theories which will remain mostly theoretical because we can't do certain experiments yet since the technology for conducting those experiments does not yet exist. For example, there are different theories about what the new Hadron Collider (particle accelerator) will tell us. In quantum physics, the new theory is called "string theory" or "M-theory." They both still have "theory" as part of the name because experimentation is still difficult to do. This is all a round about way of saying "new theories meet resistance until the community can test those theories with experimentation and offer feedback: and there will be more resistance if it goes against well established theories which were thought to have been absolute and true.
Because to accept new scientific theories can challenge the traditional interpretation of matters that previously relied on faith and religion for answers, e.g. the Theory of Evolution vs. Creationism when it was first introduced.
Sometimes accepting scientific theories means changing personal behaviors to fit the new scientific model, or involves a significant dollar cost to businesses and individuals, e.g. HIV research, education and funding, or global warming and climate change and the development of alternative energies.
There can be a number of reasons for this, but here are the two main ones, in my opinion:
- The theories can fly in the face of what people believe, or what they want to believe. For example, the idea of global warming still faces resistance because it goes against what many people want to believe.
- However, many new theories also face resistance from within the scientific community. I would argue that this is largely because new theories necessarily go against old, accepted theories. Many scientists will have based their careers on the old theories and will not want to be told they are wrong. This second point of mine is based on the work of Thomas Kuhn.
Sometimes widely accepted theories are difficult to change, even if there are alternative explanations, due to conservatism.