#2 To not take the spotlight off the women
Some have felt that the women main eventing WrestleMania 35, while well deserved, has been a little forced. While the storyline began on a great note, over time, it became a little convoluted and confused, to the point where a lot of people turned on the storyline. So now imagine if the Seth Rollins vs. Brock Lesnar match had gone on right before the women.
With all due respect to Becky Lynch, Ronda Rousey, and Charlotte Flair, it would have meant that there would be reduced hype around their main event match. After all, Brock Lesnar has held the Universal title hostage for a long time. The fact that Seth Rollins is the face of RAW is a very big deal.
WWE want as much attention to be focussed on the women's division as possible. Little wonder that the match went on first.