The REYES algorithm isn't really the problem, you can just generate an acceleration structure and shade each shading point using ray tracing [1]. The REYES algorithm is just an algorithm that generates shading points and determines in which pixels those points are visible, it puts no restrictions on how you shade those points. The downside to mixing REYES and ray tracing is that you have to keep two representations of the geometry in memory, but that doesn't mean you can't combine the two.

The main problem with RenderMan before version 19 was that it didn't have a good acceleration structure and that you basically had to implement a path tracer in RSL on your own. Pixar wrote a path tracer in RSL for Monster's University, so Disney could have just used that. I doubt that they would write their own renderer if they could have just asked Pixar to implement a better acceleration structure in PRMan, so there were obviously different reasons too.

I think Ptex was an important reason for them to write their own renderer, as they seem to put a lot of focus on textures in their paper on the architecture of Hyperion [2]. Hyperion's architecture also allows them to use packet tracing and to load and subdivide geometry on demand, which is also really great.

[1] http://www.realtimerendering.com/resources/RTNews/html/rtnv1...

[2] https://disney-animation.s3.amazonaws.com/uploads/production...