I think it's probably because they have no idea that Sasuke wasn't fully educated on what happened to his Clan at the time. So, they think that Sasuke is a heartless a-hole that just wanted Itachi's eyes or something along those lines. If that isn't the case, then they think that since he's an Uchiha, he should have seen through Itachi's lies at some point on his own. He is a disgrace because he couldn't see through the illusions, acted purely on emotion, and then turned his back on the Uchiha's integrity for a long time.