At the moment, you can add 8 GPUs to your system and run them simultaneously.  Some companies cluster multiple computers into a single image system like you sometimes see at concerts with a single image spread over a video screen matrix.

Other companies allow you to run 8 screens from a single operating system using simple photo threads with shared memory.

These other companies have been trying to convince Nvidia to develop the technology to let them use more than 8 GPUs on a single system.  Rather than expanding the capability and spending money on driver development to increase the number of simultaneous GPUs that can be used, Nvidia plans to freeze the capability at 8 because the market for more is very small and Nvidia is primarily concerned with meeting the needs of it entire customer base, not just a very few high end customers.

Basically, they are saying if the demand increases to a point that its appears profitable, then they will meet that demand as part of their ongoing business plan. Until then, they plan to use their resources to meet other customer needs.

My question is this, unless you are in the entertainment field, what would you do with more than 8 video screens running simultaneously?