Because it’s likely to prevent an intervention where it really matters:

If we are going to bomb Syria, it will have to be the way we bombed Serbia, or worst case the way we invaded Iraq: with cheaper, lower grade holy water sprinkled by the less sacrosanct NATO priests on the bombs as in Serbia, or with just some Potomac water hastily and unconvincingly sprinkled by Pentagon chaplains on the bombs as in Iraq.

But for the foreseeable future, as long as he is reasonably discreet and possibly even if he isn’t, President Assad can murder as many of his subjects as he wants with no fear that the UN will do anything about it. We stopped a relatively small scale massacre in a country that posed little threat to our interests (and from which we were getting some excellent intelligence cooperation I am told) at the cost of enabling what looks ultimately like a much larger bloodbath in a country where our vital interests are much more engaged, and whose government actively supports some of our most dangerous enemies in the region.

As Glenn often says, a replay of the Carter administration is a best-case scenario.