CGI trailers cost a lot of money actually. It would have been cheaper to put together a gameplay trailer, but they probably wanted to invest in something that looks a little sharper and has more impact.
Also if they're response for the delay was that the game design was still in iteration it would be pointless to show gameplay features as they are still in the process of being added or refined. Gameplay you see now could be completely different or gone in the next few months.
I just don't think the gameplay is ready to show yet.