The developers aren't experienced enough to make it look like that without pumping a load of resources into it. The trailer looked better because in a trailer, you can put a lot more power into graphics - afterall, you already know which path the player character is going to take and what he is going to do.
It's also that the next gen consoles are a bit less powerful than advertisments claim, but mostly it's the fact that almost every "gameplay trailer" is somewhat faked, and a lot of times the graphics are part of that. Unfortunately something we are accostumed to with videogames.
Honestly, NO launch title truly gets a good performance graphics-wise - it takes time and experience to get the most out of a console. Just look at the development of the PS2, PS3 and X-Box360 - the launch titles look relatively stale compared to the works that came out later in the console's life cycle.