Is it because of tempo?
Is it because of the length of the piece?
Is it because of the complexity of rhythms and melodic contour?
Is it because of texture (number of voices, number of notes in the chords, number of accidentals next to the clef etc.)?
I think that all of these factors play an important role here.
Everybody knows that the slower the tempo and the thinner the texture - the easier it is to play it technically.
But here's the thing - if you take two pieces of a similar tempo, would it be easier to master a longer composition with fewer parts or a shorter composition with more parts?
I understand that the answer would depend on the individual skill as well. Once you answer this question, I'm also interested in knowing why do you think it is so?