Interesting topic you brought up.
Generally, I consider that good vs evil issues in stories may indicate some degree of 'matureness'. I can't say this is a generalization but stories intended for a more 'mature' audience tend to have more complex moral issues.
Maybe it's like Harry Potter/Narnia vs His Dark Materials. The first ones [fanfics dosen't count!] have quite straightforward 'good' and 'evil' sides, whereas in HDM you're given grey characters.