A bit late to the thread, but here's my 2cp...
Just looking at the original 5e clerical domains in core books: Knowledge, Nature, War, Light, Life, Death, Storm and Trickery. Short list of fairly wide and flexible concepts each of which can be applied to many deities and religion. Take Storm for instance, apparently a very narrow concept, but which can also be conjugated to mean raw elemental forces, chaos and destruction, seafaring and navigation, weather... So each domain is meant to be versatile and applicable to many deities from different panheons (including those with a RL historical inspiration). At the same time, each deity is supposed to have clerics of different domains, so that you can create religious branches, orders or chastes: think of a deity like Apollo who might have Knowledge clerics (oracles), Life clerics (healers) and Light clerics (contemplatives).
Maybe because of that, and because each domain is not just a bunch of existing spells listed, but also a few special abilities to design, they simply did not flood the edition with lots of clerical domains like it was the case with 3e (which was of course flooded in many other ways...). Whenever they created a new domain (Grave, Forge...), it always feels a bit fuzzy-defined to me, so that you can push it and pull it in different directions. Maybe you could do that with the elements too, but they also seem a bit too dry concepts to me, not to mention that there's already plenty of possible elemental specialization for wizards, sorcerers and druids, and at least to me elemental clerics always feel a bit too alien and goofy, too "theoretical" to be ministers of faith. In fact, "Light" is the official domain that IMHO suffers a bit from the same problem.
