First off - the USA doesn't consume much Libyan oil and they don't have a pressing need for it. But let's look at something a little more important - population size perhaps? Geography? Religious strife? Neighbours? Libya and Syria don't compare at all.
Well there are many reasons why. Syria is in closer proximity to more hostile nations such as Iran which has nuclear power and Iraq. Libya is in North Africa and borders Egypt and Tunisia which have already gone through the process of eliminating their tyrannical rulers. Libya then would be an easier country to liberate then would Syria. I'm sure if things continue to get worse in Syria, NATO or even the UN would step in to try and solve the issues at hand.
