The 6850K is not a much better chip, I don't know where you got that from. The only tangible advantage is 40 PCIe lanes which isn't really worth it because AMD and Nvidia are scaling back their efforts on multi GPU and have been for a while.
The problem is the other way around really - game developers are increasingly making use of rendering features that break compatibility with the ability to use more than one GPU with exiting GPU architectures - there is only so much AMD or nVidia can do even with the best of intentions - to a degree it can be mitigated by them working closely with developers to try and work around these issues by using different approaches but these days there are less and less alternative ways at going about some advanced features.
Until developers either start making significant use of explicit multi adapter or AMD and nVidia move away from the current monolithic designs to MCM type stuff multi GPU rendering in games will likely continue on a decline.