I think a moment's consideration of history will show you that all technology goes "out of fashion" and I'd actually say guns tend to hold their perceived value longer than other types of machinery.
Wheel-locks and match-locks went out of style when flintlocks became popular. And then those fell out of favor with the advent of the percussion cap. Cartridge guns won out over muzzle-loaders. Repeaters displaced single-shots. Smokeless powder guns are more favored than black, for 90% of shooting needs. Big .45 caliber cartridges have tended to be seen as less optimal for most uses than more compact 9mm and .40s. As ORANGESI said, revolvers now hold a tiny portion of the gun market, instead of dominating as they once did. Early generation S&W autopistols were once a really big deal... and so on.
What does that have to do with Uzis and MACs? In that case, not only were both of those designs superseded by the very popular H&K MP5, but the entire category of sub-machine gun has fallen into almost complete disuse. The folks who use such things have proved to themselves, over several decades of testing and field use, that a carbine version of an M-16/AR-15 rifle can have almost every advantage that a subgun has, and several important ones that subguns DON'T have. So, almost no agency or army is issuing that type of gun anymore, the companies that made them aren't flooding the market, and movie makers aren't clamoring to feature them in their blockbuster action films.
Is a Garand or M1A
increasing in popularity? Surely not. The farther we get from their last use in battle, and the increasing degree to which they are both outclassed in various forms of competition, the fewer folks really seek them out. There were always millions more of them around and available to the consumer than there were Uzis anyway, and they're both rather inarguably more useful in general, so their lingering popularity is easy to explain.
Mosins? Really? Sure they're popular. They're the last gasp of the military surplus rifle market and are available by the truck load for under $100, and the surplus ammo for them is crazy cheap, too. Their popularity doesn't have anything to do with them being a terrific rifle, or particularly favored for any task, so much as the fact that they are ubiquitous, for the time being, and about the cheapest rifle you can shoot. That's a temporary thing and will fade as the world shifts, the supply of surplus rifles dries up (finally) and 7.62x54R ammo eventually is phased out of service.
So guns tend to "belong" to a particular era. Someone holding a Bren 10 and an Uzi (probably in a white blazer jacket with the sleeves rolled up) would be casting themselves in the 1980s, just like someone in all black and holding an MP-5 and an Mk.23 would be straight out of the 1990s. Someone in a fedora and suit, brandishing a BAR and with a Remington 51 tucked into his vest pocket would look like a 1930s gangter or G-man, and a dusty looking galoot with a hammer coach gun and a Schofield might suggest the tail end of the 19th century. Me, I prefer an arquebus, but only when wearing my best curiass.
