#3. To give The Undertaker one last dream match before he retires
The Undertaker openly stated in an out-of-character interview that he was going to be very selective of his dates and how much he wrestled. He likely felt that doing three shows last year excluding WrestleMania was too much in too short a time span.
It is true in a way, because his match against Triple H at Super Show-Down was at October, while the Crown Jewel match was just a month later. Moreover, we have no idea what Undertaker's plan is for his retirement because we've already seen him allegedly retire in 2017.
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Since they want to make the most of The Undertaker's few remaining appearances, they definitely want to give him the best match possible since he's a special attraction. It's a match that's never been done before (and one that should have happened years ago), so they probably feel that it's better for The Undertaker to face a part-timer over a full-timer.