Because Turkey wouldn't like to see Assad and his army bombed ?
I don't think Assad would have attacked Turkey if we bomb him from there,the Israelis literally bomb the neighborhoods of Damascus and they do nothing.... (Too weak to)
Anyway,we didn't want to take any risk,and it was the safest possibility.
Of course,if together (Us+France+other allies) have bombed Assad this year,he wouldn't last too much...
But thanks to their "russian brothers",it is better to deal "politically" with murderers of the 21st century than getting ride of them..
What did they want ? They wanted to get ride of Kaddhafi,if they had united after the war,maybe their country wouldn't have been such a mess....