Rare is the opportunity in life when you're given a profitable blank canvas. An empty space with your own creative spark yet to be applied, sure, but an in-motion thing that only requires maintenance and the occasional new direction, whilst making your life sustainable during the thought process? That is a truly special, treasured thing.
However, the flip-side to being granted work on an established entertainment property comes with expectation. That highly-acclaimed reputation didn't accrue itself, and usually sits atop the hard work of an entire team of hard working individuals. In gaming especially, we're often talking about 100 or more people - far more if triple-A games are on the cards.
Sometimes this notion of "fresh blood" can work wonders (just look at how much Fallout took off after Bethesda got involved), but most of the time, developers find it incredibly hard to maintain the same momentum that was there beforehand.
Quite understandably, any successful project that takes off with a smattering of characters, world designs, notable plot highs and game mechanics in tow is all down to those involved contributing to a unified vision.
Take that away... and you have some monumentally sad, disappointing misfires that most likely could've been avoided.