I think the story makes quite clear that this was meant to strike fear into the hearts of the Western governments by demonstrating ISIS's reach. That it didn't happen in the US or UK demonstrates a lot of the ad hoc, small nature of these groups.
Luke: You don't believe in the Force, do you?
Han Solo: Kid, I've flown from one side of this galaxy to the other, and I've seen a lot of strange stuff, but I've never seen *anything* to make me believe that there's one all-powerful Force controlling everything. 'Cause no mystical energy field controls *my* destiny. It's all a lot of simple tricks and nonsense.
Han Solo: Kid, I've flown from one side of this galaxy to the other, and I've seen a lot of strange stuff, but I've never seen *anything* to make me believe that there's one all-powerful Force controlling everything. 'Cause no mystical energy field controls *my* destiny. It's all a lot of simple tricks and nonsense.
