I think it's probably because people had very high expectatons after watching the first two. The first two were brilliant films, but the third lacked the greatness of the first two. I'm actually quite a fan of the third film despite its criticisms ("EW" even named it one of the worst sequels of all time). As an individual film, I would give it about a 8.5-9/10. As a Godfather film, it's probably a 6/10. But still, one of my favorites. I was much more satisfied with the ending in Part III than the ones in Part I and Part II.