Simply put, this is the reason: Tolkien has delved deeper, conducted more research, elaborated further and expanded greater than ANY other writer so far to ensure that his fantasy world would have a real elaborate history and feel "complete" and "right" to readers. Everyone knows it is NOT complete, but it feels like it is, because you know most things go so deep that it is far enough to feel complete.
And as Laurefinde says, Tolkien is a master of mixing (and switching between) adventure, action, dialogue, lore and simple storytelling, tales and songs. Everything is exactly right (for patient readers with open and (semi-)intelligent minds) and that makes it great.
EDIT: I have studied Tolkien for the last two years or so, and, together with a friend, I have just written an essay on the influence of the Old English epic poem "Beowulf" on J.R.R. Tolkien's best-known works: The Hobbit and The Lord of the Rings. The essay now contains 47 pages full of text, but we are planning to add more text on subjects we haven't covered yet and if we also add illustrations, we hope to get to 100 pages, if we can. The essay is our "profile essay" for secondary school which is kind of a last essay you need to write to show that you can do research in an orderly manner when you go to university (which I am next year).
For the people wondering: I am hoping to become a writer later in my life and to create a fantasy world that is almost as elaborate as Tolkien's. This is just a plan that I hope to execute, though... these things cannot be planned and one can only hope for those kind of dreams to come true. It's like wanting to become a professional sports player, a rock star or a hollywood actor, except that there is more opportunity for low-level starters in the writing business.
Last edited by DwarfFriend; Jan 11 2013 at 06:33 PM.
The Road goes ever on and on, down from the door where it began.
Now far ahead the Road has gone, and I must follow, if I can.