In the same way we don't say "signboard" or "billboard" but say "Kanban"? Or for the same reason we say "jujitsu" but not "gentle, soft, supple, flexible, pliable, or yielding art or technique".(see also Judo: "gentle way").
I'll take a stab at this though. The use of "jugyokenkyu" most likely expresses not just a "lesson plan", but also a philosophy and system that a literal translation to "lesson plan" can't express.