I don't think there is a monopoly renderer.

There isn't even a monopoly within Disney, they acquired Pixar 17 years ago but Disney Animation Studios still develop their own Hyperion renderer completely independently of Pixar/Renderman.

Just because it's hard to migrate existing workflows or is there something that makes Hyperion superior?

Hyperion has the advantage of being exclusive to WDAS, so they can tailor it to their exact workflow and requirements, while Renderman is a commercial product with many users outside of Pixar all with their own wants and needs.

Hyperion was designed to specifically take advantage of a specific ray batching architecture that's particularly good for out of core renderers. Which can be good for particularly large and complex scenes.