RL has tried with two ray-trace render plugins, Indigo and Iray, and both were hardly successful. Indigo required the conversion of the complete iClone scene for each frame to a local Indigo scene format, which was highly inefficient and only made sense for still images. In addition, the makers of Indigo were nor really interested in providing support for animation. Iray was a bit better in that sense, but certain things were not supported and there were issues with texturing and trees, for example.
Like Autodidact points out, each renderer has its own shading system and the conversion into that target system is not easy. So, the wise decision has been made by RL to allow exporting to other 3D applications, with exporting to Omniverse being the easiest.