(I hope I may answer this question though I am not a scholar in this field, only an avid fantasy fan who has deliberately studied the pre-Tolkien greats for a while)

I would argue that polytheism* has always been a popular element in fantasy fiction, with Tolkien and (especially) Lewis being a bit unusual.

To answer this we should first look at what religion in Fantasy was like before Tolkien and Lewis. They were after all not the only fantasists of their time, and there have always been several sources for the genre.

Lord Dunsany was one of the foremost Fantasy writers before T & L, and he set his works in a very clearly polytheistic world. His first book The Gods of Pegāna (1905) is (as the name implies) focused mostly on a pantheon of gods, and his later works also contain a multitude of deities. This charming scene is for instance from one of his most famous short stories, "Idle Days on the Yann" (first published in A Dreamer's Tales in 1910, and afterwards in several other collections):

And then the sailors knelt on the decks and prayed, not all together, but five or six at a time. Side by side there kneeled down together five or six, for there only prayed at the same time men of different faiths, so that no god should hear two men praying to him at once. As soon as any one had finished his prayer, another of the same faith took his place. Thus knelt the row of five or six with bended heads under the fluttering sail, while the central stream of the River Yann took them on towards the sea, and their prayers rose up from among the lanterns and went towards the stars. And behind them in the after end of the ship the helmsman prayed aloud the helmsman's prayer, which is prayed by all who follow his trade upon the River Yann, of whatever faith they be. And the captain prayed to his little lesser gods, to the gods that bless Belzoond.

And I too felt that I would pray. Yet I liked not to pray to a jealous God there where the frail affectionate gods whom the heathen love were being humbly invoked; so I bethought me, instead, of Sheol Nugganoth, whom the men of the jungle have long since deserted, who is now unworshipped and alone; and to him I prayed.

Another example is "Chu-Bu and Sheemish" (often regarded as one of Dunsany's best), about two eponymous gods jealous of each other.

Another pre-Tolkien (and pre-Lewis) fantasy author was E. R. Eddison. His novel The Worm Ouroborous takes place on "Mercury", but really it is a vaguely mediaeval fantasy world with inspiration from Norse sagas and Classical mythology, written in more-or-less Elizabethan English. Though one could also call the worldbuilding derivative as well as inspired; when Eddison has poems and songs, they are not written by himself like Tolkien's but rather real Elizabethan poetry or his translations from Ancient Greek. Thus we should perhaps not be surprised that the gods of his world are the Olympians (and for some reason Satan is mentioned occasionally).

When we go to early American fantasy, we will see that they are not uniformly monotheistic either.

H. P. Lovecraft is of course most famous for the "Cthulhu Mythos", horror stories set in the present day rather than a fantasy world. But perhaps it is less well-known that his "Great Old Ones" took a fair amount of inspiration from Lord Dunsany, as the expert u/AncientHistory has described here and here. Lovecraft also wrote a few stories (sometimes called the "Dream Cycle") in a more directly Dunsanian style, in fantasy lands that were explicitly polytheistic:

[On the grounds of the temples of Sarnath] gathered throngs in worship of Zo-Kalar and Tamash and Lobon, the chief gods of Sarnath, whose incense-enveloped shrines were as the thrones of monarchs

• The Doom That Came to Sarnath (the idol of Bokrug the water-lizard also makes a fearsome appearance in the story)

"Thou art a strange youth, and I like not thy face nor thy voice. The words thou speakest are blasphemy, for the gods of Teloth have said that toil is good. Our gods have promised us a haven of light beyond death, where shall be rest without end, and crystal coldness amidst which none shall vex his mind with thought or his eyes with beauty.

• The Quest of Iranon

Robert E. Howard's Conan stories take place in many fictional nations with various religions. Conan's own people appear to be polytheists* even though the only god of theirs mentioned is Crom, some examples of which are collected by u/AncientHistory in this comment. A few other gods are also mentioned (usually invoked), like Mitra and Ishtar, and various divine or demonic beings also appear in the stories. Our pulp expert has written about the influence of Howard's work on later fantasy here and here. Howard's and Tolkien's popularity has also been described by George R. R. Martin, as well as Don Rosa ("Swords and Sorcery and Rings and Things", in The Complete Pertwillaby Papers, 2012), creators who both grew up when paperback fantasy books became common in the 1960s.

When we come at last to Tolkien's Middle-Earth, there are actually few clear references to religion, something that was remarked upon at the time. When asked about it in letters and interviews, Tolkien explained that Middle-Earth is a monotheistic pre-Christian world with one transcendent God and the Valar or "Powers" as lesser beings or roughly archangels, as could later be read in The Silmarillion. But to a reader the Valar could easily be interpreted as gods, since they (and individually Varda/Elbereth and Oromë) are invoked much like deities; in one passage Theoden is even described as riding "like a god of old, even as Oromë the Great in the battle of the Valar when the world was young" (The Return of the King, chapter 5: The Ride of the Rohirrim). This is presumably a remnant of the earlier phases of Tolkien's "mythology", where the Valar were much more like a pagan pantheon. So I would think it unexpected for a writer most inspired by Tolkien to include polytheism either.

* Now I think it important to discuss that, while many of these examples are supposed to be polytheistic religions, really they seem to function more like henotheism or monolatry. In Howard's fantasy stories the Cimmerians seem to focus their worship entirely on Crom whilst not denying other gods, with other peoples doing the same with Mitra or Set. Something similar appears to be going on in that passage from "Idle Days on the Yann" I cited, and that is more-or-less also the premise of "Chu-bu and Sheemish". This phenomenon is something that has been discussed by Bret Devereaux, whose expertise is Roman military history, in this blog post. I would think this comes from living in a monotheistic society generally, and from reading the Bible and picking up the henotheistic elements, which u/KiwiHellenist have described here. Dunsany at least was strongly inspired by both the Bible and the Greek Classics, as he wrote in a letter quoted in Frank Harris' Contemporary Portraits, second series (pp. 148-9)

I hope this was helpful and not a mess, 'tis getting late and I am rather tired