Mainly because it was planned as CGI from the start, and also because they were going for a specific artistic look with the film so that even if it's CGI, it's supposed to look more like a painting. It's something that probably could be done with traditional animation, but would be much more expensive to do.

(Edit: This movie was in the works while Pixar and Disney were having a falling out, so at the time Disney was pushing CGI. It would probably destroy too much work to start all over with traditional animation.)