What Zidane has done was once what will never happen again probably.
He got lucky on many levels and I think he knows it and that is why he doesn't return to the business because he would be measured by what he has done on the past and he knows it can't be done again.
I won't be amongst those that galvanise things up, but having special players like Cristiano Ronaldo, Gareth Bale, Toni Kroos, Casemiro and probably Luka Modric helped him win all that. This is the reason why he's rebuffed all talks of managing another side. The French national team is the only side he'll be willing to manage again asides that, it's a big no no for him. That same season he won the Champions League Competition last, he was so poor in La Liga he had to resign immediately the Champions League was lifted.