This bombing was prepared by one guy, who is an expert, and executed by another, who is just an expendable patsy. The expert lives on to make another bomb. This is because there are many people in Afghanistan, and out of that large number you can find experts and suiciders. And, of course, you need to consider that Afghanistan is in a state of war for last 20 years, and a whole new generation grew up who never saw peace, doesn't know what it is for, and is comfortable with weapons and explosives.
Terrorists within the USA are, so far, stupid. They aren't bright enough to build a working bomb. Tim McVeigh wasn't stupid, and his bomb gutted a large building. Eventually AQ will send enough instructors into the USA, and once that happens the bombs will be very deadly. Those instructors may be already in the country, since the border is open for anyone willing to hike; it's even more open for those who carry weapons. The safe houses are already here (mosques.)
That’s a good answer, and I agree with most of what you say. I think your take will explain 30-50% of the problem. I don’t think it can cover the vast difference in competency. If an expert makes the bomb, any fool can push the detonator, it’s the construction that is failing.
If you go all the way back to Richard Reed, there has been an amazing run of failure. Hmmmm. Perhaps your idea about high quality fixers located in country explains more than I first thought.
Thanks for food for thought.