Most people are conditioned to be interested in Superman, especially with the right promotion, because the character has been ingrained in western pop-culture for longer than any of us here have been alive. Superman Returns did pretty good as well, despite Warner canning the series, because it "underperformed" (which, in their terms, meant that it didn't make them the Uncle Scrooge cash pool they wanted).
Man of Steel didn't fail either, it performed roughly on the same principle. It was a more flawed movie and raised some controversy on different grounds than SR, but it worked for them well-enough.
The burden lies with them, though, because "well-enough" just wasn't... well... enough for them. They wanted the biggest hit they could get and Man of Steel didn't provide. I don't think they lost faith in Superman so much as they never had it in the first place. They just took another risk, since Nolan was done anyway; I mean, they couldn't reboot Batman yet, because if they could, I highly doubt they wouldn't have done that instead of a solo Superman movie. So, when Man of Steel wasn't the home run they wanted, they did what they always do: BATMAAAAAAAAAAAN.
Snyder bears the responsibility for the quality of his product, but in the realm of stupid decisions that burden said products even further, WB is king.