This question was asked on https://github.com/docker-library/golang/issues/11#issuecomm... too, but the main reason is that the "debian" base images are tightly controlled and kept really minimal, so it's easy to make the new images as minimal as they can be too. The Ubuntu images are also great, but they include more stuff as part of keeping a consistent experience, so they don't have the same focus on minimalism that the Debian images can.
Note that the image maintainers (starting with tianon above) reserve the right to change the base distro they use for the images. So they can truly use the best tool for the job, now and in the future.
Changing the distro underneath would break all other Dockerfiles that assume they are running on a certain distro. How do we have to deal with cases when the distro is switched from debian to centos for example?.