According to a report by Dave Meltzer of the Wrestling Observer Radio, Brock Lesnar and Paul Heyman actually requested to open WrestleMania.
The Universal Championship match surprisingly opened WrestleMania 35, becoming the first time that the Beast has opened a PPV, but it was apparently Lesnar and Heyman themselves who pushed for that spot on the card.
The match was originally supposed to be much later on in the card, reportedly being booked to take place before the main event in the place that the Intercontinental Championship match ended up taking.
At around 3:30 pm ET, the match was changed to open WrestleMania and Lesnar and Heyman was reportedly pushing for that to happen as they knew that the match would be better received as the opener rather than later on in the night when the crowd was tired.
Wrestling Observer Radio