Because the P965 board (P5B) is cheaper. The 975x board (P5W) is if you intend to use crossfire. If you don't, why would you spend more?
If you're not overclocking, there's bugger all difference anyway. They just run at the same speed with very tiny differences in end performance.
In any case, I would say that the P5N-E SLI is the best of the bunch anyway, regardless of whether you want to sli or not. Cheaper with loads of cool features, and decent OCability.