I've generally understood it to have more to do with the amount of time allotted to learning, and what you have to learn in that period of time. If you have only a short amount of time in which to learn something, your learning curve is going to be much steeper than if you had a longer amount of time to learn the same amount of material. So a steep learning curve IS difficult because it means you're trying to cram six months worth of learning into three weeks, or whatever.
More material in the same amount of time would produce the same curve.