How do you even know that?
Nolan spoke highly of Zack aswell let's not forget that and he said that he himself wouldn't be able to do a big FX movie like MOS.
Here's an article from 2013 that briefly details how Snyder got the job (it's about how studio relationships with directors can backfire). However, under the link, I've copied and pasted the Snyder part for anyone who doesn't want to read the whole story:-
http://www.hollywoodreporter.com/news/internship-flop-at-box-office-566329
Over at Warner Bros., studio chief
Jeff Robinov's fierce loyalty to director
Zack Snyder is being tested June 14 with the $225 million
Man of Steel. The relationship dates to the 2007 hit
300, even though Snyder's three subsequent Warners films --
Watchmen,
Legend of the Guardians: The Owls of Ga'Hoole and
Sucker Punch -- disappointed. However, while giving him
Man of Steel (over the other finalist,
Darren Aronofsky), Robinov took out insurance with producer
Christopher Nolan, the studio's most important filmmaker (
Batman,
Inception). "Chris had the confidence in Zack, and based on the movie I've seen, Chris was spot-on," says Warners president of domestic distribution
Dan Fellman.