#3 Some of the "Deadman" aura has worn off
The Undertaker is the greatest gimmick in professional wrestling history. It has spanned numerous generations and still holds weight in the ultra transparent age of wrestling in 2020. It's hard to get things over on fans today, so part of the smoke, fog and supernatural aura was starting to wear off a little.
It's why he appeared similar to the American Bad**s instead of the classic Phenom on the WWE RAW where he signed the contract for his match. Rumors circulated after that appearance that he was trying out something different than his hugely successful Deadman gimmick. Wrestlers that have been around for awhile enjoy their longevity due to being able to adapt and switch things up when they need to. Just look at the success of Chris Jericho.
Jericho has been Y2J, the Ayatollah of Rock 'N Rolla and Le Champion. That's a super impressive list of characters for one man to use and to use in a wonderful fashion. The Undertaker has also done the American Bad*** character but has basically stuck to his supernatural guns over the years. This new character could be something different but people will probably always clamor for the Deadman. With some of that luster worn off, it just might be time to call it a career.