Itachi didn't need the MS to be threatening.. xD The guy could just be in the same area as you and put you under a powerful Genjutsu without moving a muscle....Well the manga reason is because "Kakashi didn't have the chakara required to be use it prolonged like"
but the "real" reason, is, I'm sure that Kishi didn't decide to give Kakashi mangkeyou sharingan until after the original series.
EDIT:
Plus, Itachi had to be threatening. If Kakashi or anybody for that matter had the Mangkeyou Sharingan before Itachi then it wouldn't be as "strong" and "scary"