Every publisher wants to make a big splash when revealing a new game. Being the first thing that players see, publishers and developers can often ride the hype of a great reveal trailer for years, coasting on its success to deliver a game that's guaranteed to be a hit regardless of whether the final product itself is actually any good.
However, it's no conspiracy that video games can change dramatically during development. With years being spent to craft a title that will hopefully sell millions of copies, there's plenty of amazing ideas that never find their way to the final release, either due to time, budget or creative reasons.
Unfortunately, this can lead to games that show amazing promise with a stellar reveal trailer being marginally different, and worse, when they come out.
Either changing too much or not being representative of what the promotional material suggested, there are some games over the years that couldn't hold a candle to the quality of their first trailers, completely squandering their own potential and leaving fans pining for the game they wanted to get, rather than the one they were actually lumped with.