Yes mogul skiing is difficult. oops, just remembered the "why"
A few thoughts come to mind,
Rare is the slope with both a consistent pitch AND devoid of any underlying ground changes.
Add to the above, skiers of various abilities who have kindly helped form the moguls you are about to enjoy and challenge yourself with.
Also add the variety of changing conditions along with slope rating, mogul sizes, shapes, snow cover, lack of snow cover, distance between moguls, personal conditioning and now the options encountered begin to suggest "more than" a single instructors tip or skill set.
I have read through this thread out of my passion for mogul skiing. I have viewed a couple of the videos, namely the second one from Jonny Moseley and the one from the gentleman who used the "reverse bicycle pedaling analogy". I also took a look at two of the videos suggested as good mogul skiing for pure pleasure. I did not view the videos of the skiers being critiqued.
While I have never taken a lesson, nor am I knowledgeable on many of the terms and abbreviations used in this thread (which does not allow me to write descriptively "what I do" at the same level), I am fairly proficient in the moguls. I can say that after thought and consideration, I use some of both of the above tips, but also utilize at least 2-3 additional, distinct movements or skill tips that I have developed for myself over the years which are all valuable in the negotiation of, at many times, a single mogul run.
So my succinct answer would be."Mogul skiing is difficult due to the many variables encountered all within a single run, causing countless adjustments to maintain balance and grace".
For those looking for a bit more, it's all about making your own commitment to improve, finding a good teacher or someone to emulate and the "time spent in the humps".