Because there is an even longer director's cut and a short 130 minute version which was the version initially released in America, and is incomprehensible.
The film traces the lives of four Jewish gangsters from a New York City ghetto through 60 years of 20th century history in an odd way. It focuses on three time periods - 1920 when the gang is in their teens, 1932-1933 as prohibition ends, and 1968 when Noodles (Robert DeNiro) returns to New York as an old man after he gets a letter saying his true identity has been uncovered. Noodles has been living with regret this past 35 years, because he feels responsible for his gang having been killed by the police back in 1933. He wonders if someone is planning to settle an old score with him.
The Godfather this is not. There are no family ties binding any of these characters together, and they are extremely unlikeable and only vaguely characterized. Only Noodles is humanized even a little bit, and then he ruins that by turning out to be a rapist as well as covering the requisite thief/murderer territory that comes with being a gangster.
What does it do right? The cinematography by Tonino Delli Colli captures the gritty vibrancy of New York's Lower East Side, the glitz of the Prohibition era, and the melancholic decay of the 1960s. There is great attention to period detail, from costumes to production design, immersing the viewer in each era. Then there is that memorable score. As for the acting, De Niro shows the versatility that he always does, and James Woods as Noodles' best friend and gangster ally Max plays the part as ambitious and cunning. Plus Woods always injects just a little bit of crazy int his performances.
What did it do wrong? Leone's last film has the same problem with editing that Scorsese has had with his later films. It's just too long and has lots of side stories about union bosses and strikes that add nothing to the narrative. Finally, there are a total of two rapes in this film, with one of them actually being played for laughs. Leone did this in "Duck You Sucker" and caused me to lose all sympathy for Rod Steiger's character as a result. Does Leone not get how such crimes are received in the United States?
Overall this film actually transcends the gangster genre. It's not about family or the gangster lifestyle. It's about the passing of time, guilt/regret, memory, friendship and growing old. It's also just as much a mystery as it is a mafia movie, as there is much debate as to whether or not anything that happens in the 1968 segment is even real or is it a heroin induced dream of Noodles as he tries to forget his part in the death of his friends by getting doped up in an opium den. I'd say - You decide. It could go either way.
