[j. d. worthington]

Well, there's that (which is quite accurate, in its way); there's also the fact (as it notes on the title page of LotR) that these were written by the hobbits, and therefore are more likely to be colloquial, or at least less rigorously scholarly, as they were intended to be read by hobbits, as well. Whereas The Silmarillion is supposed to be taken from older Elvish traditions, as are the majority of things in Unfinished Tales and HoME (save for the sections of the latter on LotR). Tolkien followed through on such stylistic niceties, being very aware of such things. That's also why the sections on the Annals and the later history of Arwen and Aragorn reads more in the "high style"... by that time, the material was at least based on things from sources in Gondor, even if slightly recast by later holders of the Red Book....

Which is not to say he didn't make errors in such (though they're surprisingly few, considering the extent of what he did). Simply that he was spot on in choosing the style to convey not only the overt information he wanted the stories to tell, but also the feel of the traditions from which they (supposedly) came....