The nobel prize is granted for discoveries that advance human knowledge. Whether that discovery is through chance, sheer hardwork or some linear combination, what matters is the discovery. It is not not disputed (by anyone) that the discovery was important here. The theory of the Cosmic microwave background was already out there -- it just needed someone to confirm the theory !
One could argue that the microwave background radiation was discovered precisely by the nobel winning experimenters because they refused to explain away the "noise". They were persistent and kept trying to eliminate possible sources of this noise. Perhaps other similar horn antennae had the same noise but this noise was just explained away by the other experimentalist as being an artifact of the equipment ? One may never know.
Lastly, it has been my experience that luck seems to strike competent and fastidious researchers more often than average ones.