A variety of reasons.

People tend to believe that others around them are more conservative than they are. This extends to politicians, who believe that their constituents are more conservative than they really are. It also impacts individuals, who will say that they are conservative until they are given a list of policies to rank...at which point most people want pretty much the same thing - for the government to stay out, for them to be able to do whatever they want, and for taxes to pay for the stuff we all use. Which is exactly the opposite of what "conservatives" put forth in legislation.

So it's a perfect storm of over anticipating the conservative lean, which in turn reduces the number of people willing to speak up to correct the lens, which allows shitty policies to go largely unchallenged.