[PapaBear3625]

I’m guessing that the pressure cooker acts to keep the explosive gases in check up to a point so that when they finally blow there is more force involved. Maybe this is super obvious to all anyway but I dunno, maybe not.

Alternately, the air-tight seal could contain the odor of the explosives, so bomb-sniffing dogs would not smell them.

[Oldexpat]

Someone who comes from the Muslim world is more familiar with pressure cookers. They are widely used to conserve fuel in developing countries that cook single pot foods.
They aren’t that popular in the US and sales should be easy to trace.