Players may not choose it (unless they are Barbarians), but we have these things called NPCs, who may live on low-tech worlds, who also need skills to describe what they know how to do.
Also, there could be legal or society reasons. Example in the current US, without getting into the government judgements involved...
"Black Powder" hunting seasons in many states give a great deal of extra time to hunt--at least for deer if not more species--with less hunters overall in the field. Some states also treat black powder rifles the same as shotguns for the purposes of hunting in less rural hunting safety zones.
Because the black powder legal definition is usually something like caseless muzzleloading black powder or black powder substitute, with a separate priming, the difference in our era has turned into a slower rifle to reload, with the same basic power and accuracy.
In my case I purchased a rifle with a primer slot that is very easy to load, unload and is protected from weather, fiber optic front sight with adjustable aperture rear, copper bullets with a semi sabot plastic skirt that shoots from a front and rear rest into about 1.5" at 100 yards, and black powder substitute pellets that are easy to load, very uniform, and much easier to clean. As a bonus, the rifle costs half of a basic regular bolt action rifle, even when purchased from Walmart.
So there can be reasons other than tech: social, archaic legal, no longer valid safety concerns, religious... at least in game.
As for tech, if we (modern countries at TL7/8) can make, market, and sell a modern black powder rifle for $200 or 250, when a modern bolt action costs $425 to $550 for the same basic quality level and with faster reloads, how cheap could a TL9/10 world make the same quality level black powder rifles, primers, powder pellets, and bullets, and pack them up into several shipping containers for the PCs to carry to a TL3 world?
Edit to add: I forgot, but there is a special skirt/sabot bullet with a thin rod on the bottom that holds two pellets, and you then purchase plastic cylinders to hold the bullet combo. That plus a device I didn't buy that feeds primers into a ready slot for super fast depriming and repriming, means your field reloads when hunting are really a matter of how quick you can pull the rod, push the round, and return the rod. The plastic skirt makes the pushing much easier.