A question of tone

[Morthoron]

The Hobbit was published in 1937, and The Fellowship of the Ring came out in 1954. That's an interim of 17 years. It would be logical for an author to ease previously youthful readers into the heavier material found in LoTR. However, I don't think Tolkien was ever quite "logical" in the creation of his universe. He was, if anything, confoundingly haphazard in assembling material, moving in fits and starts, writing in much the same way as he spoke, with words tumbling out in profusion.

It is more likely that Stanley Unwin, who wanted to publish a Hobbit 2.0, would be more averse to straying from the original (and highly successful) fairytale-like millieu found in The Hobbit for the more challenging, and at times very dark, plot of LoTR. So, Tolkien had to break him in, giving him the adorable and hayseed Hobbits for the first few chapters, before weaving in the far more ancient and terrifying story percolating in Tolkien's mind for several decades. Sir Stanley, it seemed, was lulled into a far greater tale.

Remember, Unwin originally declined Tolkien's submission of the Quenta Silmarillion, as being obscure and "too Celtic" -- not to mention, to paraphase Tolkien, that the "million words" was not likely to see the light, even if paper were easily available. The publisher instead asked Tolkien to write a sequel to The Hobbit.