First off, that fish cannot survive...if it is not dead already, please euthanize the fish at the first convenient opportunity. If you don't know anything about euthanizing fish, this
is a good resource with some basic info. Note that there are varied opinions in the aquarium hobby about how best to do this, my point in providing that link is not to weigh in on the current debate but rather simply to provide a starting point.
As for the diagnosis of "why," it's probably impossible to say without a microscopic examination by someone who is a fish expert. But it looks to me like either some sort of internal infection that weakened the lining of the gut, or else possibly a really really bad case of constipation that entirely blocked the fish's digestive track.