Because different writers, directors, and producers want to differentiate the character's origins or personality that they thing will attract viewers.<br />
It's a whole lot like that episode in Batman where the three teenagers have differing ways of interpreting who Batman is. Same character, but each one has a different art style, behavior, and just overall Badassness that makes Batman... well, Batman.<br />
Some source material probably does not translate well to the cartoon show and vice versa.
I havn't seen it myself but you do see the trend? It is happening in movies, television, and in video games.
Producers need to have some of those paperbacks. Actually, most of them. They don't care how it is done. Connect the way the education system(At least in the US) with the popularity of certain entertainment media.
What makes us think, gives us debate... it is not in the Entertainment industries mentality to give more of that. Why?
Because parents would rather not punish their children for acting like little brats, but instead, give in to their little kids' demands to see a PG-13 Die Hard sequel and expect the whole family to enjoy it.