Why secondary-world fantasy is normally pseudo-medieval and preindustrial and the ways in which fantasy trilogies deal with their medieval settings would be the topic of a whole other paper or papers, or a whole other book or books; I taught a course on this theme when I was employed at Rutgers at the very beginning of this millennium.
This is partly, I suspect, because of a different approach to narrative, but may also relate to the epic tendencies of modern fantasy trilogies. The medieval romance is frequently--not invariably--rambling in a picaresque kind of way.
However, hexalogy is not quite accurate, since in these examples the six-part series comprises what are actually two trilogies that have been hastily spliced together after the fact.
The incorporation of characters from the earlier movies who do not appear in Tolkien's The Hobbit (e.g., Saruman, Galadriel, and above all Legolas) provides some small sense of continuity between the dramatis personae of the two trilogies, although it invites unwanted questions.