Another reason that sales are up is that prices are low relatively speaking. The market is flooded with sub $800 AR-15’s and ammo is slowly dropping to pre 2013 prices. Many who felt “they’re comin’ to git ma guns” are selling low after buying high just a few years ago.
Guns are durable. They last for generations, given some reasonable care. Hardly anybody buys a new gun because their “old one wore out.” Therefore, gun manufacturers, both domestic and foreign, work hard to produce new models with features that will attract both new and existing gun-owners. A person who buys one handgun for self-defense in the home is very likely to buy another that is well-suited for concealed carry, then another that is good for showing off to his friends at the range, and so forth. (In my state, obtaining a concealed carry permit eliminates the need to obtain a special permit to purchase a handgun, making subsequent purchases much simpler.)
Guns are inexpensive. I’m unlikely to buy several cars when I only need one for my own use, but handguns and rifles are priced in the same range as nice golf clubs, so gun-owners find it practical to pick up a new one periodically. As others have mentioned, circumstances have combined to make guns even cheaper at this moment. Buying a new gun a year is very practical for many gun-owners.
Gun designs and technology are advancing very quickly so the manufacturers can stay competitive. (See my first point.) For example, compact 9mm handguns are extremely popular right now, but they hardly existed at all 20 years ago. Their configuration, contour, grip style, method of operation, capacity, safety mechanisms, etc., vary widely and their low price make it possible for a person to purchase them just to try them out. Many of my friends have purchased handguns for concealed carry, found “flaws” with them, and then purchased another handgun that they thought would serve them better. Only a few of them actually sold the original handgun. I don’t think I know anybody who only has a single gun.
TL;DR: Existing handgun owners buy a lot of additional guns when the guns have desirable features and are cheap enough.