When it comes to prompting new muscle growth, "different" works. For you, high reps were different from what you'd been doing, so you saw new growth.
6-12 is a pretty broad range, but in general, it has the "best" compromise between using some pretty heavy weights and still getting a decent amount of volume in (depending on total sets, of course).
I talked about one study on higher reps here:
Jim Kielbaso also had an article talking about "why" high reps work sometimes: