Cause drones can be jammed/blinded up close. It's much harder to defend from a bullet fired from a kilometer away.
The main reason those are not (yet?) being used for terror attacks is that the skill level required to pull that off is quite high, and the people planning terror attacks seem to find it easier to recruit suicide bombers than geeks.
If any of _these_ guys: http://ardupilot.org/about/team - got "radicalized", I suspect they'd be very difficult to defend against...
Radio jamming can be as broadband or narrow band as you desire. The narrower the band you target, the less power you need. The catch is, 1) you need to know what frequency you are trying to jam, and 2) this is not at all legal, so it would probably only fly in the middle of a warzone.