Same reason as setting the game affinity to 1 core in a multi core CPU can make it work while having it set to 2 cores makes it crash.
Compatibility is teh issue. And if compatibility on hte software or the hardware side is off for the game, then it wont work unless you make it compatible. In this example dissabling SLI will cause the software and teh hardware to be compatible while having a 2 card SLI network would only cause the software to hang.