It looks like Ambrose was pulled due to his injury and not some filming or other appearance as WrestleMania Week is all hands on deck. He was also pulled from several World of Wheels convention appearances this month, including this past weekend in Salt Lake City.
The RAW Superstar has been out of action since December after suffering a high-grade triceps tendon injury. Ambrose was expected to be out of action for 9 months, forcing him to miss WrestleMania. He reportedly met with doctors in Birmingham, Alabama a couple of weeks ago to get checked out.
Below is WWE's announcement on Ambrose being pulled from Axxess:
Sting and Ric Flair to stand in for Dean Ambrose at WrestleMania Axxess Premium VIP Autograph Session
Unfortunately, Dean Ambrose will not be able to appear at the WrestleMania Axxess Premium VIP Autograph Session on Sunday, April 8, at 10 a.m. However, we are pleased to inform the WWE Universe that WWE Hall of Famers Sting and Ric Flair will appear at this Premium VIP Autograph Session. Your existing Dean Ambrose Premium VIP ticket will still grant you access to the Premium VIP Autograph Session on Sunday, April 8, at 10 a.m. with Sting and Flair.
We hope you enjoy WrestleMania Axxess. See you there!