I would suggest that the main problem is that there is little contact between your beliefs and reality.
For instance people may have voted for Trump for other reasons than the idea that he was going to build a hugely expensive wall, and somehow get Mexico to pay for it. To say that Trump voters agreed with all his policies is obviously wrong, they merely need to have preferred Trump to Clinton - and I remind you that he still got fewer votes than Clinton, even so.
In reality now that funding must be provided by the American tax-payers who can blame Republican Congressmen for being reluctant to foot the bill ? And given the disruption the wall would cause along the border it is understandable that those who represent the people who will be affected would have further objections.