I feel that for any user enabled content genertion, the game must have excellent tools. Tools which are very stable and are able to hide the ip factor involved in their making. However, unless the game is intially devised to be so flexible, it becomes extremely hard for developers to open such options at later stages of development. This I believe is because the game development industry is still not very sure about the best practices of software engineering. At the same time the increasing costs of development and tight deadlines restrict most of the companies to try experimenting with newer theories.

Guess what I am trying to say is although the designers would want to have more user content generation capability for their game, the heavy dependence of this on efficient game development cycle and thus increase in cost, restricts such a move.