I read an interesting op-ed today but dang, I can't find it. The author speculated that one reason is that all the other Arab states had already decided to throw Gahdaffi under the bus so hitting on him would have little repercussion. Syria is too allied with Iran, and so intervention there could cause troubles as is to a lesser degree true of Tunisia, Egypt and Yemen. Plus, the scale of slaughter in Libya was way worse than in the other regimes, or so the author claims.