It's because although that achievement is very very good, you also have to look at the game play when you judge someone as a coach. Zidane took a great team and maintain their form (which is not easy I know), but he never shown whether he is capable of constructing a game plan, buy players that suit the plan or foster a player into one. Also to add one thing, Zidane was criticized that his tactics was bland even in his successful days. When you have Ronaldo, Modric, Kroos, Benzema, (prime) Bale, Ramos, Casemiro in your team bland tactics still win you most of the games.
TBF Pep was also questioned in this aspect but I think he has shown that he does have such capability recently with City (albeit with a ton of money), but it's still question mark with Zidane.
What? then what did SAF do all his years?