Because Cycles is a rendering engine, while FireRays is a ray intersection acceleration library. Cycles could opt to use FireRays for ray intersection if they w

Because Cycles is a rendering engine, while FireRays is a ray intersection acceleration library. Cycles could opt to use FireRays for ray intersection if they wanted to, but most uses of FireRays probably could never use Cycles since need a rendering engine with entirely different capabilities (real-time &c.)