Do we know the history of how Tolkien's work became popular?
Something that's easy to take for granted is how influential Tolkien has been for high fantasy and imaginative fiction in general. Quite frankly, many if not most of the "fantasy" tropes in popular culture either originate from Tolkien or were popularized by him. It's remarkable, then, to imagine a literary and cultural world where these do not yet exist.
The appeal of The Hobbit seems relatively easy to appreciate. It's a children's story and draws from European folk stories and mythological themes. It's wonderfully written and tells an engaging, fun, succinct, and exciting story. It's no surprise that quickly gained traction. I've read it twice to my kids and they've loved it both times. The book is a solid kid's read and highly accessible.
Lord of the Rings, meanwhile, seems like a much more remarkable success story. The book is, arguably, Tolkien's masterwork and probably one of the most influential books of the 20th CE. It's well-written and is the centrepiece of his legendarium. But it also stands on the shoulders of a children's story, is three physical books in length, includes vast appendices, meanders through verse and song, and takes a heck of a long time to resolve its story. All of that is of course incredible for fans in retrospect, but from a marketing perspective it must have been baffling. Frankly I can imagine many publishers would have dismissed it as "unpublishable" and perhaps even self-indulgent, a book so internally focused that readers must commit to the worldbuilding to fully appreciate it, rather than being able to just skip along the surface of an exciting read.
Thankfully, though, it did get published and became extremely popular. Do we know where it drew fans from, though? These days, largely thanks to Tolkien, an established "fantasy genre" readership, together with related fandoms would have a readymade market for such a book. In short, and to be crude, both nerds (like myself and probably many people here) and even just the nerd-curious would instantly understand the thematics of Tolkien if he were released into today's world, and he'd quickly find a readership. But in 1954, who was actually buying The Fellowship of the Ring?
I hope the scope of this makes sense... I love Tolkien, and the more I try to imagine fiction before him, the more remarkable his success seems.