Also to add: how did Jimmy Hoffa and his disappearance become a wide pop culture phenomenom? Many people have disappeared but Jimmy Hoffa's has been the reference to go to. Why?
I suggest to you that Jimmy Hoffa's disappearance lies at the intersection of three factors: celebrity, mystery, and death. I'll explain here what the best explanation for Hoffa's disappearance is and, in a nod to /u/cozyduck's question below, offer some educated speculation about why he remains a pop culture reference.
At the time of his disappearance in the summer of 1975, Hoffa was a national figure. Even if an American didn't follow him regularly, they certainly knew who he was. In the 1970s, the Teamsters boasted a membership of more than 2 million ─ very nearly 1 percent of the entire country. They were a political force and a social force, though I would contend they were trending toward a conservative force instead of the liberalizing force they were in the 1930s and 1940s.
The Teamsters of the time had a dark side, however. It had deep ties to organized crime. In the case United States v. International Brotherhood of Teamsters, this dark side came into full light. As part of the lawsuit's settlement, the Teamsters agreed to a trusteeship that purged hundreds of corrupt officials and resulted in the creation of an independent review board to oversee the union.
Mafia influence in the Teamsters was alleged as far back as the 1940s and 1950s, sometimes extremely publicly. The U.S. Senate Select Committee on Improper Activities in the Labor Management Field reported in 1958 that union president Jimmy Hoffa had extensive contacts with organized crime. Hoffa, speaking in his defense in front of the committee, exchanged barbs with committee counsel Robert Kennedy (yes, that Kennedy), and both gained national repute as a result. Hoffa was nicknamed the 'Napoleon of Teamsters' by the New York Times.
While Hoffa was re-elected as president of the Teamsters, Kennedy became attorney general and made prosecuting Hoffa a priority. In 1964, Hoffa was convicted of pension fraud and jury tampering, then sentenced to jail. His handpicked successor, Frank Fitzsimmons, became the union's president. After his release from prison in 1971, Hoffa began a campaign to resume his presidency, even though he was officially barred from union activity for a decade. Hoffa was fighting that ban in court and in the court of public opinion.
Busting the Mob: The United States v. Cosa Nostra devotes an entire chapter to the 1989 U.S. v. Teamsters case and gives a simple explanation of the background behind that case, including Hoffa's circumstances.
The most common theory behind Hoffa's disappearance is that elements of organized crime feared that Hoffa's resurgence would rekindle scrutiny of the Teamsters. This, in turn, would endanger the mafia's influence within the union. If Hoffa were out of the picture, the scrutiny would disperse ─ or so the story goes. Another version of the story is that Hoffa was preparing to blow the lid on the corruption within the Teamsters and thus portray himself as a redeeming force vying to clean the institution. There were no shortage of explanations at the time.
Freedom of Information Act requests by the Detroit Free Press in 2006 led to the publication of the Hoffex Memo, which details the FBI's best contemporary information and theory about Hoffa's disappearance. The FBI has subsequently declassified almost all of its Hoffa file, which is available online. Subsequent books, such as "I Heard You Paint Houses": Frank "The Irishman" Sheeran & Closing the Case on Jimmy Hoffa have also claimed to shed light on the case.
While no definitive evidence as to Hoffa's fate has been revealed in the 40 years since his disappearance, a preponderance of the evidence indicates Hoffa was killed in an orchestrated action involving people who believed they were acting in the interests of the Teamsters, and elements of organized crime. This is detailed in the FBI's own files as well as supporting non-FBI sources.
No charges were ever brought against individuals because of a lack of evidence that would have allowed the FBI to indict an individual. Despite that, there is ample evidence to point to agencies that had motivation, means and motive to kill Hoffa. Think of it this way: Without eyeglasses, a person might not be able to discern the tip of a pencil. It appears fuzzy to them. But even with fuzzy eyesight, a person can see that it is a pencil and know which end to write with, even if the tip isn't visible.
But why has Hoffa's disappearance lingered in the American consciousness?
Look to the intersection of celebrity, mystery, and death, and look to other examples. The most famous American disappearance of the 20th century wasn't Jimmy Hoffa: It was Amelia Earhart. Her vanishing act in 1937 mobilized the U.S. Navy to search for her. She certainly was a celebrity at the time of her disappearance (which remains unsolved), and her vanishing act almost certainly resulted in her death.
Before Jimmy Hoffa became a pop culture punchline, and before Amelia Earhart took her last flight, there was Judge Crater. On August 6, 1930, New York Supreme Court Justice Joseph Force Crater disappeared from a Manhattan curbside and was never seen again. His disappearance sparked a nationwide search, and after it remained unresolved, he became a punchline, with stand-up comedians occasionally interrupting their routines to ask that Judge Crater please call his office.
There is no doubt that Hoffa was a celebrity at the time of his disappearance. The New York Times television critic referred to him as one of several "media brats" in a May 1975 column. Hoffa appeared on Dinah Shore's talk show in 1974. Dick Cavett featured Hoffa on his TV show in 1973. The year before, he appeared on William Buckley's famed Firing Line.
Hoffa's disappearance was seen almost immediately as a violent one. Given his past (Thaddeus Russell's Out of the Jungle: Jimmy Hoffa and the Remaking of the American Working Class is useful here), speculation immediately centered on organized crime. That turned the case into a classic who-dun-it crime mystery, with an abundance of suspects and plot lines to unravel.
That, in the end, is why I believe Hoffa's disappearance has lingered in American minds: The same intersection of mystery and death that drives demand for mystery novels drives interest in mystery non-fiction. The same dread attraction of violence that inspires examinations of serial killers inspires a look at Hoffa's case. Eventually, given enough time, Hoffa too will fade as a touchstone. With the death of each person who remembers him, Hoffa takes another step away from pop-culture touchstone and toward the fate of Judge Crater.
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