Let me 'splain
The kid was an engineering student. Engineers, by nature design/build things which are task specific. Efficiency is like breathing to these people.
A sack of nails/junk/nuts and bolts and some contact cement would have made this a much more effective antipersonnel device. It apparently was not designed that way.
Reports of more than one device may have indicated that the busses were the target, where such a device could have been placed in proximity to the fuel tanks for an effect multiplier.
The triggering mechanism would also be tailored for the job (that engineering mindset again).
For a vehicle, especially a bus, a motion activated trigger would be likely, to catch people on and around the bus or go off when the bus starts moving. In a good design the device would have to have an arming switch, just to transport it safely.
If the motion trigger were the right type, just being at the wrong angle would close the circuit.
No problem if the arming switch is off.
If, however, the arming switch were inadvertently turned on while, say, removing the bomb from a backpack, the device would detonate.
No more jihadi.
What an insensitive comment :<}
Think I'll get myself a beer to help assuage my guilt at laughing at it.