Well the 3-round max is for waterfowl, per federal law.
For upland bird hunting, it's generally unlikely that you'll get more than two shots.
In clay target sports you generally need no more than 2 shots
So...what's left? Home defense, combat, and "action shotgun" sports
IIRC Jeff Cooper was of the opinion that if you couldn't get rid of your enemies with six shots, you either couldn't shoot well, or you had too many enemies. Ergo, a huge mag is not "required" for home defense.
For combat, a shotgun has less overall utility than a full-auto AR ( though, granted, it does has its uses)
All that's left is action sports.
Now, out of all the shotguns sold, how many are going to be used for action sports? A small percentage. It's all market dynamics. As a practiclal matter, a 10-round mag-fed shotgun has limited market appeal, so there's no great incentive to build them.
We'll save the baby-killing for EBR shooters!