Inflexible

Shotguns are often touted for their flexibility.

You can tailor your shot size to the flimsiness of the target.

You can get an ersatz rifle by using slugs.

Rifled slugs make it better than a Charleville Musket, but that's an improvement in sectional density and accuracy not trajectory.

The fallacy of this flexibility for the apoxyclips is that it's bought with specialized rounds.  You can carry dove and quail rounds for small birds, buck shot for larger animals, #4 shot for medium sized animals and slugs for when you need more range.  And every round of one type you carry precludes the carry of another type.  There's a degree of overlap with each round's effective envelope, luckily, but if you need the capabilities of one in particular and misjudged the number to bring...  You could be in the embarrassing situation of not being able to participate in a gun fight while still carrying ammo.

Plus, changing ammo mid stream can run afoul of your choke.  The best choke for a given load is bad for others and some slugs are not safe with some of the tighter chokes.

Shotguns really are specialist weapons.  They were developed to fill a need and that was killing birds.  That's a handy capability, because birds are everywhere and they're nearly all edible and tasty.  Birds are also fragile, which means it doesn't take much to bring them down.  So a single hit from a small round lead ball will do the job.

Small, fast, mammals like rabbits and squirrels share some characteristics with birds and shotguns, again, are good at bagging them.

Part of the idea that a shotgun can be THE gun comes from frontier history.  Muzzle loading history.  The flexibility offered is more real here because you were making your load's composition every time you charged the gun.

Yet... the west was won with rifles.  Cartridge rifles.

Most of the development of non-fowling shotgun ammunition comes from banning hunting of larger species with rifles for one reason or another.  The shot spread that's such an advantage for taking birds on the wing becomes a liability when hunting a deer or hog.

Then there's the range...  I mentioned that a slug gun is an ersatz rifle.  That's because it's got a trajectory that makes .45-70 seem like a laser-flat shooter.  Look up rainbow trajectory to get the joke.  Shotguns are essentially pistol range guns.

Shotgun ammunition has the double whammy of being both bulky and heavy.  A 2-3/4" 12 gauge round weighs approximately twice as much as a .30-06 round.  A loaded 12ga shotgun with 8 rounds weighs the same as a Garand.  So I'd get twice as many shots (for a given weight), comparable damage (with slugs), far better range and much better accuracy.  The .30-06 gives up the shooting birds on the wing (not really, it's actually easier to shoot flying things with a rifle, you don't have to lead near as much; the reason we don't use rifles on flying ducks is because of Rule 4).

If we go for a given number of rounds instead of weight, then we've opened up the loadout to carry other stuff.  .30-06 is considered a fairly hefty round too, .30-30 is about 80% the weight and 7.62x39 64%; meaning either more shots or more stuff compared to a shotgun loadout.  Even better, the guns for the lighter rounds are themselves lighter!  Freeing up even more weight for other things.

More shots is a good thing if you're living off the land.  A rifle is suitable or any game that needs #4 or heavier shot to take.  More than a century of the .30-30 demonstrates that.  In fact, the efficiency of .30-30 is the reason that rifle hunting of deer is banned in many areas, the hoof rats were becoming endangered.  7.62x39mm is damn similar in performance to .30-30.

The law, over and over again, is why we don't use rifles on much of the game taken; not because a rifle is a bad choice.

It is one thing to be caught out and only have a single gun available.  It's a similar thing to only be able to afford one gun.

But I thought we were talking about being prepared in advance so being caught out (you know, unprepared?) isn't a factor.

See also: This and this.