As noted earlier, Steve Austin and Shawn Michaels both had previous commitments in Las Vegas for the Shot Show.
Also, on January 10th, 2013 we posted the following:
There has been a lot of talk about The Undertaker appearing on Monday's 20th Anniversary episode of WWE Monday Night RAW. While 'Taker lives in the Houston area and has been a vital part of RAW's history, there are mixed feelings about having him appear on the show.
The general feeling is that his appearances are rare these days and that his next appearance should be to set up his WrestleMania program. With rumors being that The Undertaker will face CM Punk at WrestleMania, the feeling is that it wouldn't make sense to have him return until after Punk's Royal Rumble match with The Rock.
We'll keep you posted as we hear more.