As originally announced, Undertaker was scheduled to compete for the second time this year, against Rusev in a casket match at the WWE Greatest Royal Rumble event. Not only was this the first time that Undertaker and Rusev competed against each other in a singles match, but it was also the first casket match Undertaker competed in since 10 years ago against the Big Show in 2008. However, shortly after this announcement, Rusev was pulled from the match without any public explanation from WWE, and Chris Jericho was his replacement.
Shortly after Jericho was announced as Rusev's replacement, WWE decided to replace Jericho with Rusev, going back to the original match-up. Regarding Rusev being replaced, there was much speculation of the "bury me softly" tweet that he posted, and Taz felt that this was why he was removed from the match.
Regarding Jericho being replaced, there also was no public reason why this occurred. According to the Wrestling Observer Radio, Jericho told Vince McMahon, as a courtesy, that he was going to shoot an angle with Tetsuya Naito in Fukuoka during Night 2 of the Wrestling Dontaku event, to set up a match at the Dominion show. Just a couple of hours after notifying Vince, Jericho was told that the Prince of Saudi Arabia specifically asked for Rusev to be in the match, which is why Jericho was removed.
Per the Prince, he wanted older, more familiar talent on the card, which is why The Undertaker was featured on the show in a match separate from the Greatest Royal Rumble. Rusev does not fall in line with this request from the Prince, but Chris Jericho does. In addition, the match changed just days before, and Rusev was removed from the match, which would make it hard to believe that he was specifically requested.
This could also explain how the secret was exposed that Jericho was going to Japan, because it no longer became a secret after telling Vince. Proof that Jericho always stays tight-lipped regarding his wrestling appearances was explained by Don Callis on a recent Killing the Town podcast. Although Jericho and Callis are good friends, and Callis was instrumental in setting up the match with Kenny Omega, Jericho even withheld his Fukuoka appearance from him. Callis stated that Jericho avoided answering if he was in Fukuoka, and he had to come to the conclusion himself that Jericho was there due to noticing some things in the dressing room that were similar to when he appeared in Japan for the Omega angle.