My guess is decades ago through 'channels' the word was put out that if terrorists want to kill, protest and commit terror acts with conventional weapons, no problem.. But if they play with nukes the ME, or a portion of it will cease to exist.
With the amount of nukes out there, and the millions of $'s they have, and state sponsors these groups have there's no way on earth they couldn't get a hold of a nuke, and use it, if they really wanted to. Somehow, someone got the message out to play semi-nice or else.