What I've heard--and this is mere hearsay, I'm no medical professional--is that any attempt to use heavier weights (and thus lower rep ranges) with the small muscles of the shoulder is nigh-impossible, because the larger muscles, the prime movers, take over after a certain weight. Keeping the weight very low allows the stronger prime mover muscles to hang back while the smaller muscles carry the load.
Mike Reinold describes how this is the case for almost everyone who is just starting out with rotator cuff work:
Imagine you had a pretty weak rotator cuff during an exercises such as side lying external rotation. You can comfortably perform the exercise with 3 pounds. If I were to give you a 15 pound weight, I bet your form would be awful and you would just sling your arm back using your posterior deltoid and trapezius muscles. That is obviously not good, any time you overload a weak muscle you will get compensation. That applies to every muscle in the body.
But that doesn’t mean that you can’t slowly work your weight up to over 5 pounds as you get stronger. Why would you stop at 5 pounds? What if that isn’t challenging anymore? What is the point of that? Would you stop loading your squat at a certain weight and just sit there forever?
I routinely get patients into weight over 5 pounds when working the rotator cuff, some even in double digits. If you are compensating and using larger muscle groups, you may just not be strong enough for that weight.
So more weight might be called for after you've been doing the light weight for some time with impeccable form and are not challenged by it.