#2 No suitable opponent
The Undertaker's match against John Cena in last year's WrestleMania was rushed, had no build or storyline (except for a desperate John Cena wanting a dream match with The Deadman just weeks before the show).
For a Superstar that has made such a big impact on WWE, it was a disappointing way to tell a story at a monumental show like WrestleMania.
Will John Cena beat Ric Flair's record? A former WCW Champion thinks so HERE
There is a big chance that the same could repeat itself at WrestleMania 35. 'Taker's last feud was with Shawn Michaels and Triple H at Crown Jewel, but it is unlikely that WWE will do a repeat of that feud.
There is no Superstar that The Undertaker can put over too; things could have been different if a certain Braun Strowman had won the Universal title or was at least in the Universal title reckoning.
With his stock fallen over the last 6 months, Strowman is not the right person for The Undertaker to face at WrestleMania. I don't see anyone else who would benefit from a match with The Undertaker, while there are no "dream" matches that can happen at The Show of Shows.