I think it's the whole "vocally challenging" thing. On a side note, I talked to my 38 year old sister this morning. I got her watching Rock Star, but today she said:
"Gee, it seems really unfair that they gave JD a well known song & the others all did songs I hadn't heard of."
She did say she recognized What's Up & Kiss from a Rose. I almost died that she didn't know Paint it Black. The generation gap was showing when she'd never heard of Creep or Interstate Love Song.
Imagine how odd Creep would've sounded if you'd never heard it before. You'd assume Marty was taking some weird risks in doing "his own" version.