It has (partially, I suggest - not fully) to do with the fact that the basic founding principles of each ideology are so radically different. I don't feel like digging up references atm, but it goes back to Locke and a few others. I think we can all agree that everyone has goodness and evil in them. At the least, everyone is capable of each. The conservative worldview (and again, people are not neccesarily aware that they hold this idea, and again, it's not the only reason) starts with the basic assumption that people are evil. Not necessarily nefarious and muderous, but when left to their own devices, they'd fall into decadence and immorality. The solution is that the few good people should control and guide the rest of the world. We can see this in John Adams' toughts on how America should have been as he conceived it in his mind. He wanted to elect kings, and when the king died, a new king would be elected. This system was more complex than I'm making it out to be.
In any case, I believe that conservative policies grow out of this notion. First, smaller Government, because a large government made up of humans could not be moral since there'd be bad ones mixed in with the good. Second, those who do not share the values of the ones in power (the good) must be countered and stopped since they are evil. That being said, I'm certain Adams would have been against attacking any soverign nation for pretty much any reason (though the modern cntext could not have been predicted in his time). Third, this is why we see attempts to regulate morality and a move towards forcing one set of values upon a diverse population.
As for liberals, we tend to approach from the other side. People are inherantly good until they prove themselves otherwise. I believe this is why we tend to champion so many different causes, why we have such a big tent so to speak, and why we have trouble coming forward with a unified voice. We don't believe in a singular standard (though we're really beginning to see multiple conservative moral standards). We would rather be all inclusive than cut anyone out of the picture. This of course has its own drawbacks. Rather than crack down and regulate every little detail of someone's life, we'd more likely let someone prove they're not going to hurt someone and then clean up if they do. I personally see that as a small sacrifice for a great deal of freedom, though the one hurt might disagree at the time.
Now, all this being said...**** all this fluff. I've hardly heard any substance in any of the debates so far with the candidates for every race. I would love it, absolutely love it if people pulled their heads outta their asses and actually paid attention to whats going on. Hold the candidates and the media to task, don't give people the pass on blatant lies and stop operating on an emotional level like a child. For God's sake, actually think critically about what is going on. Don't let someone sweet talk you into a point of view because they support your pet issue while simultaneously supporting everything that is destroying you. Look at the entire platform and actually vote for something that will better you, your family, and your friends. Don't trust the politicians to give you the truth, actually look up things your own damn self. </rant> Right then...