The setlist hasn't changed enough to release a second one.
This.
Although, alot of other factors have changed. The light show now is far more solid and nailed down for each song, and the show feels alot more rehearsed and completed now. I wasn't a massive fan of the 2009 legs, for a few reasons. They lacked a good opener, the new material didn't come across with a bang, the claw wasn't used to its full potential and the overall setlist were far from exciting.
The 2010/11 legs just seem to be on another level compared to the 09 ones. A DVD right about now wouln't be a bad idea. The light show is easily the best its been all tour with the claw finally bursting to life. We currently have a brilliant opening song aswell, which all the other shows have lacked. Breathe/Beautiful Day never really did it for me and others as a typical U2 opener.
If their was to be a DVD on the horizon, then Mexico City is the perfect place. They have 3 nights of filming on offer, imagine how much material that is. The final finished product would be near perfect regarding the shot's, sound, performances, setlist, special momment's etc. They could perform 3 differen't setlist over the 3 nights, and put together one 25 song setlist using the best performances from each night. Not to mention the stadium, which is truly breathtaking.
I just hope the band realised the oppurtunity they have here, and film something.