Not only do they have a large disaffected Muslim population, there is a significant Syrian component in it, unlike any other European countries. When the arbitrary borders dividing up the ME and parceling it out were drawn up at Versailles in 1919, France got Syria.
Lots of Muslims from their North African former colonies. Wasn't aware France already had quite a few Syrians, but they've probably let in quite a few more "refugees". The Muslims that have been in France for quite a while haven't joined into French society. Remember the Parisian riots of 5 or 10 years ago?
On the flip side, France probably has very restrictive gun laws, and the French people enjoy doing the sort of things free people do. Those enjoyable things spell "opportunity" for terrorists, who don't give a @#$% about gun laws, but do "appreciate" disarmed victims.
All in all a recipe for a deadly mess if police and security forces are not vigilant while maintaining the lowest reasonable profile.