This is a long-standing tradition in the game: as a single example, Greyhawk, the first supplement to the original D&D rules, contained elaborate modifiers for comparing specific weapons to specific kinds of armor that almost every player dismissed as an unnecessary complication.
Blackmoor, the second supplement to the original D&D rules, had a whole section about "Underwater Adventures" that almost always gets lumped into this category. There are pages of underwater combat rules, the effects of casting spells underwater, pelagic and benthic monsters galore, and no small supply of aquatically-themed magic items.
Most players looked at all of this and immediately decided that they would never, ever, EVER venture underwater. The most obvious obstacle of Not Drowning was the matter of least concern, easily handled by spells, potions, or magic rings; the Monsters of the Deep were formidable, but no more so than those found in other, drier regimes.
No, it was the matter of sheer inconvenience.
The normal strategies and tactics of a typical band of adventurers would be severely curtailed in the Undersea World; the mainstay offensive spells, Fireball and Lightning Bolt, were ineffective or uncontrollable; heavy armor limited one's swimming mobility; swords, axes, and other weapons that relied upon swinging were severely penalized, in favor of stabbing and thrusting weapons like spears and (of course) tridents). Bows and slings were useless; the only viable ranged options were heavily-modified crossbows. A party from the surface had a choice between fighting in hobbles, or abandoning their precious arsenal of magical toys.
And for what? The same gold and jewels that every monster hoarded, with the added bonus of overspecialized magic items that were of very little use on dry land -- or in the underground corridors whence most characters in those ancient days spent the bulk of their careers.
All in all, it was deemed far too much trouble for too little reward. Not "risk", mind: D&D players have never minded taking crazy risks with their ultimately disposable, but they have always HATED being impaired, disadvantaged, or "nerfed". A Dungeon Master who dragged his players into an underwater adventure risked horrible retribution: I have heard stories that I hope are only an urban legend about a disgruntled gaming group that added anchovies to the DM's pizza, in order to "maintain the theme".
(Excuse me, I need a moment. Brrr.)
Alas, D&D is also infamous for failures of imagination.
There is a lot of underwater-themed material in Blackmoor -- more material than a DM might need to take a party of surface dwellers on one or two dips in a pond.¹ There's enough there to build an entire campaign around, a long-running game centered around aquatic adventures. If players hate getting dragged underwater once in a while, though, who'd want to pull their party under the waves on a regular basis?
Here's the thing: Blackmoor is also full of intelligent aquatic creatures, many of whom would be entirely viable player characters.
Why not have a whole party of aquatic characters?
Merfolk, Tritons, Locathah, the inevitable Sea Elves.
If memory serves, the section even includes underwater options for the various PC classes -- again, sections that "nobody would ever use" if they were only thinking in terms of surface dwellers descending to an especially damp dungeon.
Of course, since tabletop gaming never throws anything away[¹] (except THAC0), the vast bulk of this material was inherited by AD&D, D&D3/4/5, and Pathfinder. Despite this, in the forty years since TSR published that book, we've seen official, published D&D campaign worlds set in a Hollow World, on a dying world, in the Romulan Neutral Zone of Gods and Demons, and far more, but to my knowledge, neither the Lads in Lake Geneva nor their Coastal Successors have ever published a campaign setting based on the adventure potential of three-quarters of the surface of a typical Earth-like fantasy world.
Other than GURPS Atlantis, I don't know of any other game companies doing so, either for their own systems or during the turn of the century's explosion of third-party d20 products.[²] The SFRPG Blue Planet might qualify if you're not too picky about that line between "high fantasy" and "hard science fiction".
I have never even heard of anyone running their own aquatic campaign. I've proposed such a thing myself a few times over the decades (including a superhero variation using Champions), but each time, it's been shot down in favor of more traditional game milieus.[³] Did you really expect me to go a whole post without a single TV Tropes link?
I honestly don't understand this. Mermaids are perennial and iconic elements of fantasy and folklore -- more so than faux-Tolkien elves. The ocean is a beautiful and varied environment even before you start dropping fantasy magic and fish-people into it. The generation that turned this oddball hobby into an industry grew up on Captain Nemo, Aquaman cartoons, and The Undersea World of Jacques Cousteau.
I mean, I get why some of my other pet ideas have no traction in the gaming world; even I can see that playing a squad of inch-tall CMDF agents might have a limited appeal.
This one, though, seems like a natural.