Well, Pump Shotguns are cheap, reliable, and extremely powerful, ergo when folks suddenly feel the urge to purchase a firearm fuelled by paranoia, then reach for what they know, and Pump Shotguns have a long history of use in the Military, Law Enforcement, and perhaps most importantly, Hollywood.
They're easily recognized even by a complete novice as firearms carried by Soldiers, Police, and their favorite Action Hero on the silver screen as reliable workhorse firearms that pack a serious punch.
Sure, the AR-15 has a similar reputation, but it's significantly more expensive, is constantly demonized by the media as an evil weapon used only by criminals, and has less dramatic/decisive effects when fired in movies.
Seriously, in the movies the guy with the shotgun almost always hits his mark and with devastating results , whereas the guys with ARs are either constantly missing or shooting at something like a Tank or a Monster which obviously takes no damage from it.
Even I use a Pump Shotgun for Home Defense, albeit a less conventional one.