A question of tone

[Mithadan]

The Hobbit is, unquestionably, intended for a younger audience. While the tone gets darker as the work progresses, it remains light. The trolls border upon comedy relief. Yes, Gollum threatens to murder Bilbo but there is the almost humorous riddle game and the lost buttons. Yes, there are goblins, but their portrayal does not approach the evil and more graphic images seen in LoTR. Yes, there is a dragon, but his interactions with Bilbo border upon the whimsical. The darkest images drawn in any detail are the burning of Laketown and the aftermath of the Battle of Five Armies (the battle itself is barely described).

The transition from The Hobbit to LoTR is very smooth. The tone of the opening chapters of The Fellowship is very similar to that of The Hobbit. The absurdity of the Shire and the Hobbits remains. The Shadow of the Past is darker, perhaps, and covers more serious history and events, but perhaps not significantly more so than An Unexpected Party. The Black Riders appear but are nebulous and not as threatening as they later are portrayed. Their "sniffing" for the Hobbits is emphasized, but never explained and is not mentioned later. The flooding of the bath in Crickhollow, the greed for mushrooms. The Hobbits are attacked by trees. Bombadil is... Bombadil. It is not until the Barrow-Downs, the assault on Bree and the attack at Weathertop that things get truly dark and serious.

We know that Tolkien rewrote LoTR multiple times, literally cover to cover, before it was published. One can assume that the tone in LoTR's early chapters and the smooth transition from The Hobbit to The Fellowship of the Ring was intended. Any thoughts about why?