Actually, there are two good reasons why conservatives don't, and shouldn't, compromise with liberals. One is that compromise is based on trust, and conservatives simply don't believe that Democrats can be trusted. Back in the early '90s George H.W. Bush campaigned on a pledge not to raise taxes. The Democrat leadership, however, managed to convince him that if he would ignore his pledge and raise taxes they would agree to cut spending. Both sides would anger their base, to be sure, but we'd achieve a lower deficit and maintain fiscal sanity (those were the days), and the country would be better for it.
Well, Bush agreed to raise taxes, breaking his "no new taxes" promise and all but ruining his chance of reelection, but the Democrats never reduced spending. In fact, with the increased tax revenue they decided to increase spending. The memory of being bamboozled by the Democrats makes todays conservative Republicans very leery of liberal promises.
The second reason is that conservatives believe that compromising with liberals is a losing proposition even when they don't immediately get swindled. This is because of the ratchet effect inherent in any compromise between one party which wants to maintain the status quo and another which wants radical change.
Here's how the ratchet works: Consider a series of numbers from 1 to 10 representing the various positions on any issue. The national consensus is, let's say, at 4 on some issue, but liberals want to take it to 10. So they propose a compromise that would move the issue to 6. As soon as conservatives agree to this "compromise" they've already lost the battle and will soon lose the war because we, as a society, are now moving in the direction the liberals wanted to take us all along.
Some years later, the Democrats propose another compromise. They propose that we move to 8. If the conservatives agree then we've moved almost completely to the liberal goal. Conservatives have gotten nothing and liberals are almost home, and of course there's no going back to 4. Anybody who suggests a return to the status quo ante is labelled a reactionary. The process continues over time until the nation is finally at a 10. If conservatives refuse to go along they're called obstructionists and do-nothings.
Here's something else: Even when society has been moved to 10 on a given issue liberal/progressives cannot be satisfied. The 10 is now the new status quo, and if progressives are satisfied with the status quo they're no longer progressive. They must, in order to be a progressive in good standing, continue to push the envelope ever further to the left. Because of the ratchet what was unimaginable yesterday, becomes a fringe view today and a mainstream view tomorrow. Meanwhile, those who think we never should have moved away from 4 in the first place are called "extremists."
Take gay marriage as an example. When the gay rights movement first got under way it was a movement that asked only for tolerance of an "alternative lifestyle." Americans agreed, but then the request for tolerance morphed into a demand for acceptance and then into a demand for approval. Once that was largely achieved we were confronted with further demands that gays be allowed to marry. If the ratchet ever crosses that threshhold, which seems inevitable, what'll be left for a good progressive to fight for?
Right now campaigns to make polyamory legal seem way out on the fringe, but once gay marriage is codified polyamory will probably move much closer to the mainstream. Even further out on the fringe are those who wish to remove barriers to consensual sex between men and boys, but once polyamory has been ratcheted into place pedophilia will not seem so bizarre.
What's bizarre, after all, is simply what we're not used to. As Alexander Pope wrote:
Evil is a monster of such frightful mein, that to be hated needs but to be seen. But seen too oft, familiar with her face, First we endure, then we pity, then we embrace.When one believes his position is correct, whether on any moral, social, or economic issue, compromise is an abandonment of his principles. If politicians are willing to betray their principles just to appear "reasonable" to the media then they've also betrayed the trust the people have placed in them and should be voted out of office.