Apparently FFC never wanted to make Part II either... but I've come to accept that what FFC says depends on how he is feeling that day.

The bottom line is that FFC and Puzo didn't exactly have the motivation they had back in the 70's. They knew this was a cash-in, and approached it as such. That doesn't stop the movie from having flashes of brilliance, but overall, it's a sad end to a great trilogy.