Racial bias, most likely. Vulcans in the 22nd century displayed an extraordinarily egotistical opinion of themselves (even more so than in future generations). And like many scientific communities today, when the supreme body of geniuses insists there's no such thing as time travel, and declare it to be "illogical", most other scientists will simply tow the line, as T'Pol did. Especially when the alternative is to accept the theories of an illogical, emotional human.
The same reason that scientists hang on to the theory of evolution. Also, the theory of relativity might be proven wrong too as physicists have observed particles traveling faster than the speed of light which relativity says can't happen.
T'Pol eventually accepted time travel as a possibility in ENT: Azati Prime, and she said that she changed her mind about time travel after she went back to Detroit in the year 2004. She was a little skeptical about that, but she thought it was possible when Archer brought up Daniels. She believed that he was actually from the future.