Again, to someone with no training, what is the difference between a moderately and an extremely challenging piece? I'm not sure if I can tell, beyond a certain level; all I can say about pieces is "I could sight-read that", "I could sing that with a lot of work and practice", or "there's no way I can sing that at this level of training". I'm sure that the repertoire of pieces in the third category is huge, and they're not all the same difficulty level, but I'm not sure I could tell the difference if I heard them sung.

Also, a piece that's extremely challenging isn't necessarily catchy. People tend to react emotionally to songs they know, not obscure-but-difficult violin solo pieces.