What [China and Russia] were surprised and upset by was how Libya negatively affected their interests. It did not bog down NATO powers in a costly war and all the spoils of victory appear to be going to the NATO powers. China and Russia want to avoid a repeat of that. They also can get away with that because there is no credible threat at the moment that the West intends to take forceful action to end the violence.
Paul Bonicelli expands on that point:
For Russia, Syria's dictatorship is its last client left standing in the Middle East, both political and economic since Syria provides a warm seaport and buys Russian weaponry. To watch it fall means ceding the field largely to the U.S. and the EU, and losing revenue. The stakes are indeed high for Russia. For China, the best explanation is inertia; China defines its national interest — apart from its freedom to engage in commerce wherever it can — according to the principle of non-intervention. Its reaction to the Syria situation is like its reaction to every other such situation: everyone should mind his own business, we like things as they are. (That China seems to contradict itself when it comes to Filipino, Vietnamese, or Japanese territorial interests requires a little semantic gymnastics.)