If WMD justifies war with Iraq...it would also justify war with Syria. US forces were engaged in Iraq and Afghanistan at the time.
Attacking a third muzzie country (expecially one allied to Iran, with us already in two countries that flank Iran) could have touched off all sorts of nasty outcomes.
Sometiems pretending one problem doesn't exist....prevents that small problem from becoming a very big nasty problem.
Syria has always been the Israelis backyard. Let them handle it. The took care of that nuclear plant in 2007(Operation Orchard)