Cleaning. Pumps don't need much cleaning. A little oil and it will always work if you need it. And when you do want to clean it, it's quick and easy, since there's no gas mechanism.
Simplicity. Pumps have few parts. This means they can be made both reliable and cheap, a difficult combination to achieve with a semiauto. There are no O-rings to replace, gas systems to adjust, etc.
Price. The semiauto costs a good deal more. For example, an 11-87 Sportsman costs almost double the price of an 870 Express. Guns are of similar quality and construction.
Ease of use/training. If you want your SO to learn to use the thing quickly for HD, a pump is about as simple as it gets. There are few controls, the action is visible to the shooter, and if you take a few minutes to show someone how it works, a pump makes sense to just about anyone.
Safer unloading. It's pretty simple for someone, even without much experience, to unchamber and eject the round in a pump without chambering another one, without fumbling with controls.
Proven design. The two pumps you are likely to buy, the Remington and the Mossberg, are both designs that have been made and sold by the millions. The Mossberg 500 is the more recent design, dating from the early '60s
. The 870 was first sold in 1950. Both have been used by countless cops and hunters, and keep selling as other guns come and go. You won't be getting a questionable POS with either brand of pump, though many will extoll the virtues of one variant over another. The 590A1 is arguably the best, or an equal 1st place, for defensive use.
Parts availability. You can convert your American pump into just about anything with a barrel from Cabela's, Cheaper Than Dirt, or your favorite other retailer. This can theoretically be done with a Beretta semiauto, but not as easily or cheaply.