The move to create their own development studio occurred when Disney saw that it was spending a lot more than it wished when it was licensing out characters and content to third-party developers. Disney has spent over $200 million in the past two year acquiring or starting up game development studios, a sum that the company is expecting to double over the next two years.
So why go exclusively with Nintendo? Nintendo, apparently, is a natural partner since its users tend to be a bit on the younger side. Combine that with Nintendo's approach of easing everyone into gaming with their console the Wii and Disney and Nintendo appear to be quite the match made in heaven.