The original formulation is based on a skill acquisition vs experience chart. On such a chart, a steep learning curve is one where skills are acquired quickly, as opposed to skills that are difficult to acquire. The common "steep learning curve" = "difficult" usage is understandable, given that steep hills are more difficult to climb. It's a knee-jerk reaction for me so it's transient: you're not going to see steam coming out of my ears :)
https://en.wikipedia.org/wiki/Learning_curve