A seminal part of the Evangelical mindset is a feeling of persecution. Subcultural Identity Theory tells us that what makes Evangelicalism so successful is that it simultaneously defines itself as embattled against and forced to engage with mainstream culture. Evangelicals feel as if they and their values are under attack, but as much as they would love to retreat to seclusion (à la the Fundamentalists), the Great Commission demands that they intermingle with their wouldbe enemies. (Weber had a related concept he titled worldly-asceticism: be cautious of the world around you, but whatever you do, don't cloister yourselves! )
I think that one reason Evangelicalism and Conservativism seem like such easy bedfellows is that the Conservative narrative is remarkably similar to the Evangelical. Conservatives see themselves and their values as being under attack, and in a very real and measurable sense, they are. (I'm not convinced that Evangelicals, though, are threatened in any real sense.) The march of history--of the modern project--has been progressivizing. The overall trajectory of the human story has been one of forward change. And, for those who had it pretty good with the status quo, this relative change in status can be terrifying. (Look out, rich white Protestant men! We're coming for you!)
I'm not arguing that the beliefs, philosophies, and values of Evangelicalism and Conservativism have always been the same. There are, in fact, several places that they are at odds. However, they share a definitionally defensive stance, and sometimes posture without message is enough.
I came to this conclusion reading a comparison between the tones of the speeches on the Tucson atrocity from Pres. Obama and Sarah Palin. "Where she stressed the importance of fighting for our different beliefs, he emphasized our need for unity...." Calls for unity are easy for those who are not embattled; calls-to-arms are more natural for the entrenched.
See here and here for related posts.