Part of the reason The Undertaker, Bret Hart and other big name legends weren't used on RAW last night was because "Stone Cold" Steve Austin and Shawn Michaels weren't available. When it was discovered that Austin and Michaels couldn't be a part of the show, the call was made to drop any plans of using other big name legends, most notably The Undertaker and Bret Hart. This was done because WWE didn't want to have a lot of legendary names on RAW and it draw attention to the fact that Austin and Michaels were not in attendance, especially considering that the show was in their home state of Texas.
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.