There are way more explanations than that. For example pollsters might simply not be talking to conservatives.
If you don't have a representative sample of the population your poll will be inaccurate.
If you don't have a representative sample of the population your poll will be inaccurate.
Do you have any reason to believe that pollsters and other statisticians are so ignorant of their own field not to understand this?
So they think they have a representative sample, but they really don't.
It can be explained simply by pollsters mainly living in large cities, and not really understanding rural Americans.