I dont know, probably plenty of people disappointed or angry about the cancellation of Firefly still. Additionally when something is cancelled there is a some closure in knowing that it will never be finished. But with ASOIAF there is hope dangling in a thread that it one day will reach a satisfying conclusion, but the longer it takes the more resentment starts to grow from the possibility that it never will. The longer hope exists, the bigger the disappointment if it doesn't pan out.