Obviously amateurs. Metal trash cans are Faraday cages, and cell phones don’t work inside them. That’s why only two went off out of the seven bombs planted in trash cans.
Faraday cages - good point.
You know for sure the trash cans were metal? I heard one witness describe the trash cans as plastic barrels. It makes even more sense as they are typically supplied by rental vendors who also provide the rental port-o-lets for big events. They are far lighter and can be far more easier to store and longer lasting than metal ones.