"Why do people still disagree with Special Relativity"?
Why do people still disagree with the theory of Evolution? You see, the average person is not hardwired to have critical (even more, scientific) way of thinking. Relativity puts you in front of facts and gives a theory that contradicts the average person's ''common sense''.
Similarly, even though evolution stands on a much simpler idea, it is rejected by many due to prejudice. In the Reativity's case, common sense plays the role of prejudice.
For example: People for years saw the sun go up and down every day. They concluded that the Sun revolves around the earth. That became part of the collective ''common sense'' of most people for millennia.
People for certuries saw species of animals producing offspring according to their own species. They concluded that species are fixed. Again, that became part of the collective ''common sense'' of (practically every) people for millennia.
So, now Einstein comes and tells us something which contradicts our inherited common sense. He proves it, thus it is evident that our ''common sense'' is invalid after all. Since it is obsolete, common sense in that case now is as good as prejudice.
Same principle applies for evolution. It became documented with loads, with TONS, of evidence, but our faulty 'common sense', in addition to religious centuries-old indoctrination, stood again as an obstacle to scientific, accurate learing of the world around us.
So, as I like to say, ignorance is an option. When you see that your ''common sense'' is contradicted by facts, and yet you continue to hold your position, then you prove that you choose to remain ignorant.