[Mumak o' Harad]

I think that The Lord of the Rings was written under the Hobbits' perspective, and maybe it is, in fact, narrated by the Hobbits; therefore, I think that the books "grow up" with the Hobbits over the course of the story, i.e. from the innocence of the Shire (childish) to the dreads of The War of the Rings/Mount Doom (adult), and the literary style accompanies the Hobbits' development throughout the story.

Have you noticed that the Hobbits are not present only in few passages of the books (like when Aragorn, Gimli and Legolas are on the errand to find Pippin and Merry)?

We have to keep in mind that in the end of the story, even The Shire is totally changed in view of Saruman"Sharky"'s control (not so innocent anymore), and the story only really ends when Sam is safe home: "Well, I'm back!"

In the end I think that Tolkien, being the perfectionist he was, would not have changed styles without a clear purpose.