Gritty realism is a sort of rejection of romantic fantasy. And it resonates well with modern western culture, specifically young males whom have grown up in an era of sarcastically overplaying the tropes of old. Making the classic tales into parody. A meta reaction to the idea of lighthearted mysticism.
It's not just a matter of appealing to the largest market. It's a sort of self fulfilling prophecy. People want these things because that's all that's marketed because that's what companies believe that's what customers want because that's all that's marketed because that's what companies believe that's what customers want because that's all that's marketed because that's what companies believe that's what customers want yadda yadda yadda.
In time, a rejection of this culture will happen, and grown men will start to be fond of rainbows, kittens, unicorns and sweets and neon clothing will become a common sight. Then that culture will be rejected and so on, and so on. History on repeat sense the beginning of art.