The reason for this is that D:OS uses a deferred renderer; those can handle much more lights on the scene than traditional forward renderers, however they render in multiple steps, and each step requires feedback from the previous rendering step.
While forward rendering could be parallelized relatively easily on multiple cards, as there was no feedback from the GPU, this is not the case for deferred renderers, so they cannot really utilize multiple GPUs.
I believe games only supporting one GPU will become more common, not less, as most new games/engines utilize deferred rendering.
(eg. Unreal 4:
https://answers.unrealengine.com/questions/21746/does-the-ue4-engine-support-sli.html)
