In short? Budget.
If you agree that a role-playing game is the ideal framework to tell a story that promotes Christian values, then you would have to assemble several talented staff members (writers, artists, programmers, testers, etc.) to create a RPG that would reflect positively on Christianity.
Development costs for larger-scale titles continue to increase. A smaller-scale RPG on a platform on which it's less expensive to develop games (e.g. Nintendo DS) might be a more feasible approach, but there would still be some very difficult obstacles along that path, including the current state of the US economy.
I don't doubt there are Christian writers and artists out there who could make an amazing game that glorifies God (some of them may already be working in the industry, as my previous post suggested), but bringing them together and convincing them to work together on a new video game at sub-standard salaries (which would likely be necessary as it may would range from difficult to impossible to find investors for such a venture) strikes me as extraordinarily difficult.
I'm not saying it's impossible to create an outspoken pro-Christianity game that's also fun and engaging to play, but there are several obstacles in the way.