The reason is far more disheartening then "republican politics" or whatnot. The problem is that Syria is in the heart of the Middle East, and it serves western interests far more effectively to allow a strict dictatorship there than for it to be plunged into civil conflict, like Libya.
The problem is that Syria, like Libya, is filled with discordant factions that are being kept down by dictatorship: Extremist Sunnis, Extremist Shias, Outspoken communists who up until 15 years ago were being paid for by the USSR. Syria was also a Soviet client state, but now is relatively neutral. American involvement would most likely contribute, or succeed, in deposing Assad, but the Baathists are not going to just roll over and die in Syria, as we can clearly see from the present blood bath/detentions that are occurring. Thus, rather then a stable, if thoroughly undemocratic state, a US intervention would simply create more chaos with no resolution in sight, in an area of the world already filled with lawless regions. Syria would become another disaster zone like Iraq quickly.
Recently, Syrian citizens have actually been half-jokingly suggesting that ISRAEL should intervene. I think, in a real way, this sums up the ACTUAL situation on the ground in the Middle East, as opposed to the American view typified by Ladies in Black, that Israel is somehow a devilish Nazi-like state. In fact, the COMMON MAN in the Middle East knows that Israel has a better standard of living, more trustworthy representatives in government, and many of the amenities of the West that none of them have. Rather then "hating the Jews," many Arabs and Persians ENVY them, and that is something the Ladies in Black just don't get. They confuse envy with hate, and "side" with those that are envious!
Despite these glimmerings of real change in the Middle East, the chances of either Israel or America actually entering Syria are nil, as Syria ALREADY represents a hostile power WITH the capability to if not destroy Israel, at least to seriously harm it. Israel has refrained from attacking the nations around it for the main reason that any wars against these powers outside of Israel's borders would be either prosecuted with mass murder, mass deportations (to where?) or both. Israel, while it may be ABLE to do such a thing, is guided by religious and political principles that don't allow it. So, instead, they have built a massive defensive system, and part of that system involves maintaining control of the Golan Heights and at least certain areas of the West Bank and Gaza. But, because these are basically No Man's Lands between warring enemies, those that live there (the Palestinians) get very short shrift from ALL parties involved. Palestinians cannot (at least not until this new wave of chaos) enter Egypt, Jordan, Lebanon or Syria. Like the Israeli checkpoints, Palestinians are abused by the other powers as well.
In conclusion, it is not up to Israel, the US or anyone else, to CONTRIBUTE to the chaos surrounding the VERY poor choices these governments made concerning their own peoples. It is not Israel's, or the US's fault, that 100,000's of folks are dissatisfied with their oppressive governments. It would serve no purpose to rush into Syria with an Israeli/US army, as it would not only bring even more chaos to Syria, but could inspire Iran to military adventurism. We must wait and watch, and grit out teeth, unless, we, as a people, are willing to put ALL of our society's resources to the mettle. If 75 MILLION of us decide that we NEED to save the human race, starting with Syria, then ok, but if anyone is expecting a few 10's of thousands of troops to solve this problem, they are delusional.