It's due to personal fear, really, and a couple other things.
Nowadays, everyone wants almost, or full-on paranoid control over their work, characters, and stuff like that. While I can understand not wanting certain imagery being associated with a character for some people. That's part of the reason: I fear getting backlash from the original author of a character. While I don't draw X rated pics pretty much at all, I still fear the backlash because of fear of not portraying the character the way the author wants.
It's something I struggle with every time I think about drawing fan-art of any person's character. Another thing is the absurdity of having to ask permission to draw fan-art period for some people. It's a concept that's completely asinine as much as it is restrictive. People draw fan-art all the time for major franchises like Mega Man, Mario, Sonic the Hedgehog, MLP:FiM, and more, yet nobody has to, or does, ask for permission to draw those characters.
Granted, they aren't doing it for profit, but that's how most fan-art is by nature: free. It's also free publicity, free recognition, among many other things. Restricting people from drawing fan-art of their characters means the only one getting to draw the character is the author, and while that means they have complete control of their work, it also means nobody is getting the chance to get in on the fun of giving those characters an existence. Nobody gets to contribute to their work without a "Yea" or "Nay" from the author.
Most artists thankfully don't operate this way, but a good number are starting to, and it's a sad, sad reality.