Eh? Where did that come from?
Tolkien was writing during the pulp period -- in fact, the majority of his published fictional work comes from that period. It just doesn't come from the pulp background, which is an entirely different thing.
Again, I have to disagree with you, especially when it comes to popular writing. Writers for the pulps had to (to a greater or lesser degree) bow to the demands of the editors and, beyond them, the readers, even when it meant going against the grain artistically, doing things they knew were not good for the sake of the story, in order to keep those paychecks coming in. That's just a fact of the business of writing for popular outlets -- something Tolkien didn't have to concern himself nearly as much with, while Howard made his bread-and-butter that way.
Which leads to the subject of Howard's prose.
I'm sorry, but I really must strongly disagree with you on that. Not that Howard wasn't capable of writing very good prose (and frequently even better poetry), but that, as I note above, due to haste and occasional carelessness, plus his isolation from wider exposure to higher grades of literature (and education), his prose was also frequently slipshod at best. It was almost always entertaining, but it was
not always well-written. Again, Howard himself was well aware of this, and bemoaned his faults (and did try to improve -- he did care about his writing, most definitely), and so were his contemporaries.
Again, Howard's characterization is, by his own admission, kept simple and direct. As he put it, he wrote heroes who, when in a jam, are "too stupid to do anything but cut, shoot, or slug themselves into the clear" -- this from a letter by Howard himself. That in itself becomes a severe limitation rather quickly. Tolkien, on the other hand, allowed his characters to be much more complex and nuanced, which in turn allows a much richer emotional connection, on more levels. He also did not limit himself to a particular sort of style or prose, but varied it according to the needs of his tale and the other things he wished to convey. While Howard also used varying voices, he did not have near the range JRRT did, in part due to the simple fact that he was a much younger writer, and his experience in numerous realms of life much more limited.
All of these things have a great deal to do with how good a writer becomes, and Howard was dealing with a lot of limitations Tolkien simply didn't have.
Again, I disagree here. Lewis, while often quite good, could also be rather hasty in his writing, and many of his things are simply not thought through, so that it becomes easy to spot errors and contradictions now and again; much more so than with Tolkien, who took time to very carefully craft his prose to convey precisely the subtleties of thought and feeling he wished to convey.
Lovecraft doesn't yet have that big a reputation in standard literature, though he's getting there. Chandler, on the other hand, has had something of that for a very long time. There are good reasons for both to be in the positions they are, and one of those which looms largest is simply that Chandler writes about things that more people can relate to both emotionally and intellectually than Lovecraft, who can be (for many people) a very difficult writer. For another, Chandler's prose is much simpler, more direct; whereas Lovecraft is much more old-fashioned, more nuanced, textured, and aiming at extremely subtle effects a great deal of the time, while appearing on the surface to be doing just the opposite. Lovecraft also, as Joshi noted, combined prose-poetic techniques with the classic essay form; a very unusual combination requiring careful attention from both writer and reader. Chandler, on the other hand was in some ways a follower of the Hemingway dictum "Kill your darlings". (Not entirely, no. There are passages in Chandler that are anything but; but on the whole it's a statement that applies.)
Tolkien lies somewhere between the two. While his prose is almost never as dense as Lovecraft, it is also seldom as spare as Chandler could often be -- but not never. Again, it depended on what he was striving to convey, as he chose the tone and voice best suited for a particular effect -- something a careful reading of his fantasy as published during his lifetime will reveal, and which is even more obvious when one reads the books of his work edited by his son. He was an incredibly painstaking craftsman -- much more so than almost any writer of his era I can think of, with the possible exceptions of Lovecraft and Cornell Woolrich, or perhaps James Branch Cabell.
As for HPL -- as I recall, your only experience of him (at least, the only one I recall seeing you comment on) was "Polaris", which is scarcely typical in many ways. Very Dunsanian (even though written before he had ever read Dunsany) rather than in HPL's own "typical" voice or manner. You might try The Case of Charles Dexter Ward for something more "mainstream" from his pen, or "The Colour Out of Space" for a glimpse of something that is truly uniquely Lovecraftian....