Zidane will never top what he did at Madrid, knowing the massive amount of luck involved. It doesn't make sense for him to take another job and be found out; he's done the unthinkable, nothing left to prove.
Xavi, on the other hand, has something to prove, but he wants to be put in a situation he thinks he can succeed in, but those opportunities aren't being given to him because clubs see other managers as being better suited for the job.
I think he'd rather not manage than go to a situation where he thinks he's set up to fail, because that would be it for his managerial career.