... why, out of all of the 'Tolkien-clones' that the publishers crank out, very very few of them even begin to approximate the original.
The short answer goes something like this: Because Tolkien was a brilliant man who was doing something that did not have any existing parallel, and everyone who came along since has simply aped the results from the outside.
Most people reading this are probably at least passingly familiar with the story behind the story. Tolkien developed the Rings as, at least in part, a container for his personal work in developing a mythology and an artificial language, and his independent studies in religion and philosophy. It also hadn't meant with unanimous praise; Michael Moorcock (ever the leftist in his critiques) and David Brin are two of its more recent and nameworthy critics, and it took decades to reach anything like critical mass in its name-recognition and commercial success.
Once the mass-market paperback editions appeared and started selling fiercely, two things happened. One, after a couple of decades, you had a whole generation of writers whose formative experiences were the Rings. The book had always existed for them. The same goes for the publishers, who always had the Rings as a constant reminder of what was possible. If someone else could create something of that stature ...
Everyone who's managed to create an original product ends up having their work picked to pieces by people who are looking for some way to duplicate its success. The list of things that got copied from Rings could fill another book: the various races and their constituents, the naming conventions, the use of magic, the quest template, the nature of the heroes, the wizened wizard, the three-book cycle, etc., etc. And so we started getting the endless flood of books that copied one, another, or all of these things in some form. Maybe not deliberately — some of the most shameless plagiarism is committed unconsciously — but both the publishers and the authors got stuck in this despairing feedback loop. The harder you try to write a classic, the less likely it is you'll get one, because then you're playing a loser's game of trying to second-guess history and anticipate future taste.
I suspect a good many fantasy authors have not heard of Harold Bloom's The Anxiety of Influence, so it might be instructive to talk about it here. The book deals with precisely the above problem — how do you get out from under the shadow of your precursors? — and while it uses poetry (Shakespeare/Milton) as its example, it could apply to most anything else. Everyone starts by copying their predecessors, he argues, and if I were to argue from my own experience I'd add that it's inevitable and a deeply vital step. I spent years writing horrible cheeseball manqué ripoffs of all my favorite authors, and it was only after a lot of shredding of paper that I wised up and moved in different directions. It's not a question of whether you do it but how long, to what extent, and what lessons you glean for making a clean break from such mimicry.
This is the hardest damn thing in the world.
It's not a surprise that so many have copied Tolkien, badly. It's not even a surprise that it's gone on for so long. What's surprising is how few people on both sides of the page are willing to cop to it and do something about it.
The best tribute to Tolkien's intellect, craft and creativity would be to go and do something so unlike his work that he would have no choice but to applaud it.