What if it's not a bomb - but something else? Say, Bio or chem weapon concealed in a laptop, that is otherwise fully functional? Or software to hack the aircraft?
There's only a few reasons for stashing suspect devices in the hold, and potential explosive power is not one of them. Either the threat requires the attacker to actually use the device (hacking the a/c), or the device contains something that would be lethal in the cabin, less so in the hold.
The earliest reports said the ban would last 96 hours. That's a very precise time frame.