Source: The Wrestling Observer
One of the major reasons that Vince McMahon has been supportive of CM Punk keeping the WWE Title for so long is because of The Rock. McMahon was said to be very supportive of CM Punk holding on to the title at Survivor Series because he's worried if the title gets put on someone different, The Rock might have an issue with it.
CM Punk vs. The Rock at Royal Rumble has been teased since the summer and is something The Rock agreed to do. Vince reportedly doesn't want to upset The Rock by changing things last minute regarding the creative plans for 2013.