I think one of the reasons is that after the Iraq war - when the French stood up to the USA and refused to support their illegal invasion, the USA have pressured the French into supporting their military action, and recently the French seem to have taken a lead (out of the European countries) in bombing places like Syria, Libya and (I think, but correct me if I'm wrong) putting troops on the ground.
I also believe that France has always had a problem with racism and there has always been an issue with the amount of people from ex-French colonial countries living in France such as Algerians, Moroccans ect so I suppose it's a melting pot of racial tensions which, during an economic downturn, is easy to magnify those feelings in angry, disassociated young men and to turn those feelings into hateful rage.
I personally think that things will get much, much worse for Syria and the ME after attacks like this, but then you can say that their countries are getting blown up by Western forces already, so some uneducated, angry young men may feel that striking back - even if it's in the most immoral, evil way against innocent victims, is their only way - despite all logic which says otherwise.
