not really delayed, as there never really was a set time that they were going to release it. they have alot of video to edit through, audio to edit, bonus features to throw together... a demand for a Nevermore DVD has been pretty big among their fans, so I'm sure they're going to do what they can to make sure the "Final Product" will be worthy of anything a fan would want.
That could be it... or it could be the decision to put a song that is a COVER on the DVD.. It's fun live, sure, but it's not a good representation of what the band is.