They used to be popular before we invented better solutions - first revolvers and then semi-autos.
Rifles are mostly used for precision shooting and double barrel guns are not ideal for precision at longer ranges. Not only are the barrels offset, but each barrel is slightly different and may have a slightly different point of impact.
Pistols are meant to be small and light. Two barrels make a pistol larger and heavier, unless the barrels are small (see derringers). Nowadays you can get very compact semi-autos and revolvers, so derringers are out of fashion.
And TBH pump-action and semi-auto shotguns are very popular too. Double barrel are used for hunting and specific sports like trap shooting. Some people also own them for home defense and such, because:
- they inherited it
- other guns are illegal where they live
- “nobody needs more than two shots” (until they do)
- they are cheaper than pumps and semis
- Doom Guy
There are also single-shot shotguns which, again, are used mostly for hunting.