#2 The Undertaker and WWE are not on good terms with each other
The Undertaker is taking non-WWE bookings for the first time in his career. After a long career with WWE, The Undertaker is looking outside WWE for bookings.
He is involved in the Starcast II and will be appearing there. As a result, his status with WWE authorities is not at the best level.
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Thus, Undertaker did not appear at WrestleMania 35.
#1 The Undertaker will be appearing at upcoming Saudi Arabia pay-per-view
The Undertaker is not in the best of health. He is old, and there are only so many appearances that he can make.
WWE might have a pay-per-view in Saudi Arabia on the 7th of June and according to reports, the Undertaker is booked for it.
As a result, he might only be appearing at Saudi Arabia which ensures a better payday. This is why he skipped WrestleMania 35 and was not available for the event.