That's because it is way too simple to mess things up, all it takes is a hammer and you can probably get a 50:1 kill ratio. (the number of people one asshole can kill before being killed themselves).
Terrorists are the 'griefers' in real life. They need cooperation from the media to give their deeds the attention otherwise there would not be enough incentive for them to go through with it. Hence the trend towards the more spectacular but less efficient ways of killing people.
If wholesale murder really was their goal they could do a whole lot better than they're doing, let's hope they don't catch on to that.
As long as there are people willing to have their minds poisoned by 'authority figures' without thinking for themselves and taking responsibility for their own actions this problem is unlikely to go away.
A better approach would be to limit the media exposure to the dry facts and give proportional attention to the deaths of people that die in less spectacular but equally tragic ways.
CNN is just a big a factor in a terrorist attack as the victims are, and as long as that's the case this sort of attack will continue.
Imagine hearing 'a terrorist attack four months ago killed 12 people' versus live footage from the scene of a blocked off times square within minutes of it happening.
The first would result in a shrug, the second keeps nations glued to their tvs, gives the terrorists more recognition and therefore an incentive to continue.
We're way too fascinated with this terrorism thing taken the amount of people actually killed on average per year, it would be much better to direct the funds and effort going in to all this to more productive - but less politically sensitive - issues.
Poverty for example kills a lot more people per year than terrorism ever has, or ever will.