President Barack Obama drew a proverbial "Line in the Sand" claiming that "use of Chemical weapons crosses a red line." Now that line has been crossed, with confirmed use of the chemical weapons by Bashar Assad against rebels seeking to oust him. Now Obama has to back up his words with action, lest the world see him as a "sitting duck" that they can ignore.
President Obama has drawn no such line on Egypt, consequently he doesn't see any mandate to take action there. His inaction though, shows how deeply conflicted he is about the situation in Egypt. He wants to stay on the winning side, so he can establish friendly relations with the government that emerges, which is why he refuses to recognize Morsi's ouster as a coup, that could put him in hot water if a military dictatorship emerges.
Our President really needs to put his foot down in both places. He needs to have the US Navy Seals extend a "Zero Dark Thirty" invitation to Assad, just like they did Osama Bin Laden (Either Assad leaves peacefully with them, or he dies by firing squad). He also should declare that Morsi's removal does constitute a coup and yank all funding from Egypt until the sand settles, and we know what the Egyptian people have chosen.
Of course, any answer by those of us not in government is only speculation. However, I would argue that there are three factors that cause this to be the case.
First, for whatever reason, we see weapons of mass destruction as a special case. We tend to focus on the nature of the weapon rather than the number of people killed. This does not necessarily make sense, given that it seems just as atrocious to kill someone by asphyxiating them in a police van as it is to use chemical weapons on them. Given our sensitivities, it is understandable that we are more likely to intervene in Syria than in Egypt.
Second, Syria has not been friendly with the US in recent history. In Egypt, we are at least somewhat tied to the military government. By contrast, Bashir Assad has never been a US ally.
Finally, it seems much more clear as to who the “good guys” and “bad guys” are in Syria. No Americans have any real reason to support Assad’s regime. We may not like all of the rebels, but we clearly know that we do not like Assad. By contrast, it is much messier in Egypt. We do not like the idea of military rule, but we also have pretty negative views of the Islamic Brotherhood. Even the majority of Egyptians do not like the Brotherhood.
For these reasons, we are, for better or worse, more likely to intervene in Syria than in Egypt.