Why do you think that? You don't think the setting of Waterdeep and the particular factions and Realms elements that implies will be integral to the adventure? And if not, why would WotC bother nailing it to Waterdeep so solidly in the marketing? Have any of the previous adventures been generic enough to convert to a setting as specific as Eberron (or, say, Darksun or Planescape) on the fly?
Because it's not that hard.
I've been converting
Tomb of Annihilation to my homebrew fantasy world on the fly. And previously did the same with the 4e adventure
Madness at Gardmore Abbey.
Realms elements will be included. But the Realms is generic as eff. It's easy to keep most of those and just add Eberron flavour.
I expect elements of Waterdeep to be tied to the adventure. But I also don't expect it to be impossible to convert.
The trickiest bit is flipping proper nouns on the fly. Gods, city districts, and the like. But a half-hour of prep and couple index card cheat sheets clipped to your DM screen make that effortless.
Locations are easy. Just find a parallel. Sea Ward = Dura, Dock Ward = Cliffside, Castle Ward = Central Plateau, Trade Ward = Tavick's Landing, South Ward = Menthis Plateau, North Ward = Northedge. Likely not perfect, but that took 30 seconds... Groups and organisations are trickier, but only slightly. Most common groups have easy established counterparts that serve a similar function. It's easy to replace the Xanathar's Guild with Daask, and the Masked Lords just become the City Council. For those without a comparative match, just move the group over wholesale.
Other details like shops, NPCs, and the like can just be used as is.
The descriptions and boxed text will need to be dumped. So the DM will need to paraphrase and insert Sharn elements and flavour. That requires a little more invention and creativity. But less than making an entire adventure yourself.
To answer your other question: why would WotC bother nailing it to Waterdeep so solidly in the marketing?
Because it's better to have some default flavour and setting than no setting and require DMs to invent a city on their own. The Realms is a placeholder rather than generic names (like early 1e and BECMI effectively was) or a setting that is made up for adventure (like 4e).