Presumably they want to appeal to a wider audience than just the fans of the show/comic/etc so they can pull more money in, so they feel they have to take a different approach to the material, as well as try and fit the entire story into a 2 hour or so framework as opposed to episodes or issues.
As for competence, that's a different matter altogether.

Because they aren't that familiar to the material and didn't put their blood, sweat and tears into the original works (
and are only in it for the money, really), they don't really understand what to do... just all about thinking what wider audiences want.