I think it's simply because vast majority of people are pretty OK and don't feel like killing others or blowing things up. No doubt there will be some attempts somewhere at some point, but generally - if people were to be eager to make drone-bombs, they'd probably be using RC car bombs before, or simply shooting at others more often. It seems however that in all but the most chaotic places, people generally stick to more peaceful ways of voicing their concerns or disagreements.
Someone has done a proof-of concept for this at a Russian press conference, by which I mean that they buzzed the VIP table with a drone carrying a large-ish dildo (which I assume weighed about as much as a small explosive payload).
I'm surprised that no one has done it with regular RC planes or mortars. It's not like new technology is necessary for this.
Heck, why not just throw the bomb in a trash can, or hide it under a car in a parking lot, etc. It's not like bomb placement is the difficult part of terrorism.
My thought is that this is simply a matter of time (and initiative and opportunity).
High-level targets, say, top leadership of G8 nations, would likely be difficult. But attacking key field leadership, business executives, or heads of state of smaller nations is well within current capability.
A drone buzzed Angela Merkel a couple of years ago (video on YouTube), and a Russian leader more recently.
My sense is that a small directed weapon (effectively: gun) would be more effective, though an explosive charge and shrapnel could certainly be utilised.
The technological advantage the US and other advanced nations have in drone technology is quite likely to be short-lived. Especially if your interest is, say, 1-5km range, rather than 100-500km.
Some time ago Secret Service posted a work order[0] for software that could detect (among other things) "sarcasm and false positives" in social media content, so someone is most likely working on it.
I'm just actually surprised that nobody has fixed some explosive to a little drone and flown it into some political target and detonated it.
Or a bunch off these landing on cars or buildings and detonating.