The answer is that John handles Greek at a very elementary level, with no
sense of Greek style at all, not even of Mark's rough-but-clever calibre.
Luke, on the other hand, handles Greek with an astonishing ease. While he
is of course no Sophocles (NOBODY else is a Sophocles except Sophocles!),
he can write Greek that sounds like LXX (Luke 1-2), like popular story-
telling (the longer parables in Luke 15-16), or like an exciting adventure
novel with travelogue (like Acts 27-28). Luke-Acts is a pleasure to read
in Greek, while John (as Greek) is pretty boring and tedious to my ears.
Even Mark is pretty zippy when compared to John. (I'm not saying Mark's
Greek is "better" than John's, only that his style is more interesting.)
Now, the real answer will probably come from Carl, to whom I will totally
defer in this matter.
Edward Hobbs