Because Nuno is tied inextricably to Mendes, and Nuno has spent a lot of money through Mendes to achieve nothing except line his pockets and the pockets of his huge entourage - something we've had numerous managers do and I don't think repeating something over and over again expecting a different result is a good idea.
I'd give Benitez a chance purely because he has a reputation that could attract players, he's known to be competent in the past tactically, he's won things and there's a chance he'd have something to prove. Is he my number one choice? No. Would I prefer him to dross like Howe, Potter and Nuno? Absolutely.