Purist fanbase trying to latch on to old classic Superman and thus not allowing the character evolve like other characters who have become very popular today like Batman or Iron Man, thus creating stagnation. This created the general perception of Superman being the boring boy scout that some people seem to think of him.
I take Man of Steel as an example, I personally think it was a great movie and It introduced a more modern and grounded Superman to appear to a modern audience. I know many people who didn't like or care much about Superman before the movie but became fans of him after the movie. However, purists and hardcore fans of classic era Superman grumbled and whined that "it's not my Superman"