Main events often feature shocking twists or dramatic returns. At WrestleMania 41, CM Punk, Seth Rollins, and Roman Reigns closed Night 1 because Paul Heyman turned on his allies to side with Rollins. If WWE has nothing similar planned for Cena and Lesnar, the match may not justify the final slot.
A Gunther return, for example, would have elevated the bout, but if no such surprise is scheduled, the decision to place it earlier on the card makes sense.