Why? Because if liberals believe something, "conservatives" must disbelieve it. If liberals support a thing, "conservatives" must oppose it.
I know that sounds simplistic...but it is in large measure factual. "Don't believe anything we don't tell you" has been right-wing media's drumbeat for the past 40 years. As a direct result, no matter how right or accurate we liberals are on an issue, "conservatives" feel duty-bound to claim we're 100% wrong.