Because Kryten's spare head doesn't know how to properly operate the time drive, instead they are brought to Dallas, Texas on the 22nd of November, 1963.
Accidentally, they stop the assassination of the current United States President John F. Kennedy by knocking his assassin Lee Harvey Oswald off a 5th floor window of the Texas Book Repository. To escape the FBI gunmen who think the Dwarfers had attempted to assassinate the president Kryten jump forwards in time where they find that Dallas is deserted. The crew find the body of Eric White, and later a newspaper that allows Kryten to extrapolate what their influence on history has been. The consequences of this was that in 1964, John F. Kennedy was impeached when his legendary womanizing is discovered, resulting in the biggest scandal in that version of American history. He is sentenced to an open prison in July 1965, resulting the mafia controlling the U.S. via their puppet president J. Edgar Hoover, blackmailing him with a picture of him in a transvestite orgy.
The Soviets are able to successful install a nuclear base in Cuba, resulting in all major southern U.S. cities being fled from, by November 22 1966. The Dwarfers find this all out when, after using the time drive to escape the police, they arrive in an deserted Dallas. After trying to get back to their own time they find that they are marooned since the Soviets landed on the moon first, due to the U.S. being traumatized by Kennedy's impeachment. Thus the history of human space development changed and thus no equivalant of Starbug was ever built.
Kryten as it turns out knew something like this would happen all along, however because Lister removed his guilt he didn't care enough to prevent it. To fix this, the team realize that they must figure out how to re-assassinate John F. Kennedy! Their first attempt is to get Lee Harvey Oswald to the 6th floor to prevent their earlier selves from knocking him off, but that results in the same effect, but a different cause due to the sixth floor not being a suitable vantage point for the shot. Lister finally gets and idea and uses the time drive to meet Kennedy while he is being imprisoned and, in order to restore his reputation in history, and with some persueding gets him to assassinate himself on the grassy knoll (and create a scenario 'no' crime buff will ever solve). History is restored...but the others then gang up on Lister and pummel him for all the hell he put them and the universe through for the sake of a curry.