The hawks know (or thought they knew) what sharp is -- a scorer. And they paid him as such. At ~6mill a year (if i'm not mistaken) they expect him to produce like a top 6 forward. Simply put, he hasn't. It's not even like he's just "cold" or anything either, he's taken veeeeeery few good shots and gotten very few good chances. For the most part, he's been invisible imo.
Bickell, on the other hand, was signed for his ability to be the "tough guy" (and of course, his ability to transform into an allstar in the playoffs). He's payed less (~4mill a year?) and was never expected to be a top 6 forward. Ever. Now has he produced like he should? Probably not, but he's never been asked, nor should he be asked, to put up the numbers sharp is capable of putting up.
This convo is moot though, as both will be gone next season.