That's because the point of the comparison is not to compare with other renderers, it's to check feature correctness. Mitsuba is widely used for that purpose.

Almost no one uses LuxRender or Cycles commercially to the best of my knowledge (or appleseed for that matter). Hobbyists use them, which is cool.

I'm in Los Angeles and work in the industry. Here, people use Renderman, Arnold, and in-house stuff mostly. I don't think Renderman and Arnold are used much outside of VFX. Arnold in particular is pricey, but works amazingly well.