The United States and other NATO countries have been bombing ISIS in Syria and Iraq for over a year. The problem is that you can't win a war with air power alone and until now very little public interest and support existed for "boots on the ground" in the Middle East. In 2014, it took an entire summer of ISIS led genocide in Iraq (5000 Yazidis killed in Sinjar, 1700 non-Sunni Muslims killed in Tikrit, and many smaller scale killings) before the US put the coalition together that is currently conducting aerial bombardments. And even then, this was only to prevent the potential loss of 50,000 Yazidis who were trapped in the Sinjar Mountains in August 2014. That would have made the US look bad as they created the power vacuum that ISIS has filled in Iraq. More than likely the 20 bombs that France dropped on Sunday would have been dropped by one of the coalition countries (US, Jordan, France, UK, others) anyway, but the intelligence and "opportunity" were given to France so they could appear to be doing something in the wake of the tragic Paris attacks. Again, just more political posturing. If the West was truly interested in defeating ISIS we would have had troops on the ground several years ago. Instead, we have relied on proxy war (arming rebels) and non-ground attacks (aerial bombardment) as these do not require much political approval. Now that civilians in Western countries are up in arms over ISIS we will likely see troops on the ground.

tl;dr: Politics. A ground war is unpopular until a domestic attack happens (or fear of an attack), then the public supports war and politicians can send in troops and still be re-elected.

Source: mostly my opinion