To put it bluntly, the problem is that he billed it as Godfather novel, and it was mostly about his own characters. It might have been better received if it was a "Tales from the Godfather" type of story, revolving around other minor characters that we aren't aware of from the movies/novel, but referencing the Corleones. Instead it was billed to be ABOUT the Corleones, and it wasn't.

To add insult to injury, Winegardner committed the gravest of all sins in writing: He used a Mary Sue character and to make matters worse he took established characters with established personalities and had them act completely contrary to how they were established to act in canon.