#1 He doesn't need the title

Lesnar is certainly a credible Champion but he doesn't need the belt to prove his worth; he is an attraction by himself and can have matches without the Universal Championship.
It would make more sense to have Lesnar as the 'hunter', as opposed to the 'hunted', much like how the Goldberg was used for much of his WCW career. Lesnar doesn't need to carry a title and can still be used as a destroyer, who stars are lining up to try and beat. There could be a focus on Superstars trying to end Lesnar's undefeated streak, rather than have him hold a Championship he doesn't need.
Dominik Mysterio made an outrageous comment recently. More details HERE
Make no mistake, Lesnar is arguably the most credible wrestler in the world to hold Raw's #1 title but given his current status in the company, it makes no sense for him to have it and more importantly, he doesn't need it.