That is easy, they don't!
Canon have a superb ability to make what people actually want, Nikon make the things they think people want. The difference is subtle but very important...
This is something that I've been trying to explain for months.
The 5DIII was targeted to a very specific audience – wedding and event photographers. That doesn't mean it isn't a fine camera for general use, but rather it was designed and marketed with a specific professional audience in mind. Wedding photographers are about the only large population of professional photographers remaining today and they are in a very competitive market where small differences are critical to gaining a competitive edge. The 5DIII was designed to offer a competitive edge (clean, high ISO).
On the other hand, I have been unable to identify any professional market for the D800. Again, it's a fine camera, but what competitive advantage does it offer? I suppose if you are in the business of producing wall- or billboard-sized prints the extra resolution of the D800 offers an advantage. But the market is miniscule compared to the wedding and event market. And, for many professionals the high resolution and resultant files sizes can actually be a disadvantage, given that the end-use of the majority of images today is the web and not "art" prints.
Canon was able to charge a premium for the 5DIII because they knew the professional market they were targeting would pay the premium in order to secure a competitive advantage. As "private" says, Canon made a product that people actually wanted.
For products like the 6D and the D600 the differences are less significant because both are targeted to consumers, not professionals, so they are sharing and competing for essentially the same customers. Canon did their market research and determined what features and price point they were comfortable with. Nikon did the same.
If someone feels that Nikon better fits their needs than Canon, have at it. No manufacturer is ever going to produce a product that meets the needs of 100% of the market 100% of the time.