UFC is bad at promoting their fighters, the fighters don't have the means to easily market themselves or build brands like boxers can because they're stuck in predatory contracts.
Boxing could dominate the PPV landscape, but it's too spread out with too many rivaling promotions. Boxing doesn't even have to put all its best guys out there, if you had a card featuring just 4-5 big stars the numbers would be crazy.
AJ vs Fury
Crawford vs Boots
Loma vs Shakur
Spence vs Thurman
Haney vs Matias
Mbilli vs Caleb Plant
This would easily sell 1.5 - 2 million ppv buys without breaking a sweat, and it's not even bringing out the big guns.
Boxing will have the bigger stars for the foreseeable future, Davis vs Garcia did 1.2 million buys with a 22 million dollar gate. Conor is the trump card, but he's on his last leg (pun intended).