Because it's not one of the first cape movies, the novelty is gone for mainstream audiences and the already low standards that were set with shit like Captain America 1, Iron Man and Thor have been raised slightly higher by TWS and CW. There's no room for mistakes because these people have seen it all already. They think they know what's good or bad for these films.
I mean look at Cap 1, that tiny Chris Evans was silly as shit and no one ever mentions it. A good chunk of Iron Man's plot is improv, and Thor...I can't even remember that movie. DC is an unknown, you don't know what you're gonna get with their movies. Whether it be Snyder slo Mo out the ass, or an almost music video.
Marvel is a safe bet almost always, it'll be lighthearted, joke filled, and it won't ever be that serious. They made their mistakes when people were forgiving, when Cape was fresh out the oven.