Of course, not being Mafiosi ourselves, we can only speculate. The realtionships among underworld crime figures and organizations is something that is pretty much opaque to us. We really don't know what influences them and the variables they encounter and consider. Apparently, there has never been much or any of a war between the various ethnic crime factions. That's probably testament to their recognized sheres of influence and acknowledgement of each other's territories. But, as TB points out, FFC and Puzo took what they had, what they believed the audience was generally aware of and tried to (the Pizza Connection) and mold it into a story.