Google search provides us with cca 50 million results for the article version and 28 million for the no-article variant. Is there any actual difference?
It's a slight difference in connotation. "A good reason" implies "a single good reason," it implies that there is or is not a compelling reason so there's the sense of a particular reason.
"For good reason" implies that reason is a bulk quantity, like sugar. There's some measure of reason.