Because the attrition rate is one hell of a lot higher attacking our military than our 'soft' targets. Have you not noticed that we have not been hit in the U.S. since 9/11? With our withdrawals, this is going to change and our enemies will not be making the same mistakes that they made the last time. (They hit us too soon and they did not hit us hard enough)
This war has not really begun properly yet. We are losing right now. It is only a matter of time before it blows up again.