#3 He's mainly been Universal Champion for much of the last three years
Since the brand split happened in 2016, the longest reigning Universal Champion in that period was Brock Lesnar. After defeating Goldberg at WrestleMania 33, he held the title for 503 days the first time. Those were some tough times.
Once Roman Reigns defeated 'the Beast' for the title at SummerSlam in 2018, things appeared to be getting back to normal. But, Reigns had to relinquish the title, and in early November of 2018, Lesnar won the title again in Saudi Arabia thanks to interference from Baron Corbin.
While he thankfully lost the title to Rollins at WrestleMania 35 after a reign of 156 days, he now holds the Money in the Bank contract. Since he's primarily been associated with the Universal Championship, it would make more sense to have him go after the other brand's title. If they want to put a title on Lesnar, it would at least be different than giving him another run as Universal Champion. He has held the WWE Title, but not recently.