Actually it was because he won the contest. If memory serves me correctly, the Puzo estate sponsored a "Write the next Godfather Book" contest. I'm not sure of the exact name of the contest but it was an open call to any and all writers to submit their entry for the continuing story of the Corleone family. I may be wrong but I seem to remember something about a contest of some sort...

Edit - Found it!
http://www.ew.com/ew/article/0,,784837,00.html