Comment by pixelesque

It comes down to how "coherent" the rays are, and how much effort (compute) you want to put into sorting them into batches of rays.

With "primary" ray-tracing (i.e. camera rays, rays from surfaces to area lights), it's quite easy to batch them up and run SIMD operations on them.

But once you start doing global illumination, with rays bouncing off surfaces in all directions (and with complex materials, with multiple BSDF lobes, where lobes can be chosen stochastically), you start having to put a LOT of effort into sorting and batching rays such that they all (within a batch) hit the same objects or are going in roughly the same direction.