The reason the Undertaker vs. John Cena match at WrestleMania 34 was so short
WWE reportedly realized months ago that The Undertaker (due to his age and injuries) would not be unable to work a long match at Wrestlemania 34, thus deciding to make the John Cena vs. Undertaker contest an “unadvertised” attraction, with Cena only teasing a match with the dead man and begging him for a fight on WWE’s biggest annual stage. That way fans wouldn’t expect the usual long Undertaker match if they weren’t even sure he’d be there.
source: Wrestling Observer Newsletter