The title refers to it as a "craze." I don't think using the synonym "fad" adds anything. The question is "why this fad, now, rather than a different fad, now?"
Christopher Nolan’s dark night trilogy proved that these films can make money and be critically acclaimed.
Also is it really a craze? Christopher Reeves Superman was pretty popular so was Spider-Man and the Hulk when I was a kid. There were loads of movies and tv shows in the 80s and 90s with super heroes. I actually can't remember a time when they weren't popular.
The only difference now is Marvel is raking in at the box office. Is anyone else actually making money? DC aren't.