Ten Ton Hammer has an article about the differences between developing a game based upon existing intellectual property
and the creation of an entirely new story and setting. They make the point that while doing the former may result in an easier time building a fan base, those same fans will often be the hardest to please.
"By creating a game based on a popular IP, the company in question has a huge responsibility to 'do it right.' Unfortunately, not everyone realizes the reality of one little secret — every single fan out there has a different idea of what 'right' is. ... Lord of the Rings is a perfect example. For a person that may be familiar with the movies and little else, it's a great game with an impressive amount of depth and attention to detail. For the mass of fanatical fans that have spent more time poring over every book Tolkien ever wrote than even Tolkien himself, any deviation from the lore of his world is paramount to sacrilege on the most horrific scale."