No official explanation AFAIK, but here's my crystal ball...
Clerics have too many subclasses. When the design decision was made to base cleric subclasses on domains, this was kind of going to be inevitable, but the expectation is that every God of X has an X domain cleric somewhere out there potentially. And there are gods for everything in D&D. There's no Arts domain, there's no Beauty domain, there's no Dwarf domain, there's no Agriculture domain, no Hearth domain, no Wealth domain, no Moon domain, no Sea domain, etc., etc., etc.
That's just a lot of content and pagecount and development. So in the 2014 release, they whittled it down as much as they could, and they still had SEVEN subclasses. And jettinsoning four elemental domains in exchange for, say, a Tempest domain, was probably a good trade. In the 2024 release, it's been circumscribed even further (just four domains!).
Since 5e's launch, new subclasses in general are rare and the cleric's have been pretty focused. I bet there's working versions of each of the four elemental domains in house there, and they just haven't made the cut on the products when jostling for the new cleric subclass spot (especially because you'd really want all four of them at once).
The closest official stuff is probably Tempest (2014) for air or water, Forge or War or maybe Nature (2014) for earth, Forge or Light for fire. Water probably gets the short end of the stick, there - having a true Sea domain would go a long way.