I say it's pretty much just because of price.
Think of the Mossberg and Remington as the Accord and Camry of the gun world. While the Browning and Ithaca are like the Cadillacs. Sure the Cadillac is better, but most people aren't car enthusiasts just like most shooters aren't shotgun enthusiasts. All they want is a shotgun that will not break in half, will go bang when you pull the trigger and will hit what you point it at. And frankly, while the BPS and Model 37 are beautifully crafted guns. The Mossberg and Remington will do those 3 main things just as good for a fraction of the price.
Also availability. How many Ithacas or BPSs do you see on store shelves compared to Mossbergs and Remingtons. People pick the latter just because that's what's in front of them at the moment.