Because most gamers don't care much about value for money, and they buy the nVidia boards to run SLI. People will still do that (look at the amount of people buying 780i boards, and look at the amount of people who bought dying 680i boards), but they are getting fed up with them. I haven't actually recommended any nVidia board based systems for a long while now. If nVidia make a good board, I will recommend them. Simple as that. It is actually arguable whether they sell more boards than Intel do. You will find a lot of people, who don't ask for help on forums, will buy Intel boards more than the nVidia ones.
Intel boards, on the other hand, are generally more stable, better overclockers, run cooler, and don't break down. Again, I am recommending Intel boards at the moment because they do run better than the nVidia boards.
The only real exception to this are the 750i boards, which are great. But, again, they don't sell because of the lack of support for tri-SLI, and the fact that the ATI cards are the better bang-for-buck cards than the new GTX2xx cards.
