Why Lincoln's assassination is so famous is mostly a result of Lincoln's fame. Polls of both American historians and the American public regularly rank Lincoln as one of the greatest presidents in the country's history (a Rasmussen poll in 2007 of the general public gave him a net favorability rating of +88, only beaten by Washington). In addition, he presided over one of the most studied periods of American history, the Civil War. Practically every schoolchild in the US learns about Lincoln at a fairly young age as a result. His assassination is more of a footnote to his presidency, even for people without much historical knowledge, than the defining feature.

The JFK assassination is a more interesting question, and one that's hard to answer without drifting into speculation.

Certainly, the Kennedy assassination was covered more in popular culture than the others; in addition to books and films centered around the assassination (such as Stephen King's 11/22/63, and its more recent tv adaptation by Hulu), there are literally hundreds of discussions of and references to the assassination across multiple media, far more than there are discussions of McKinley or Garfield.

Of course, that's not really an answer to your question, since while popular interest is driven by popular culture, the reverse also holds true. One explanation is that there was simply more media to drive the cycle in the 1960s than in the 1880s or 1900s. News about the McKinley or Garfield assassinations would have spread over days or even weeks, primarily through telegraphs and newspapers (although some of the rich and powerful would have found out within hours, by telephone). By the 1960s, however, radios and television sets would have been common throughout America; news of the assassination spread in hours, not days, and was covered extensively on tv over the ensuing weeks; news about the assassination, about the funeral, and about the murder of the assassin by Jack Ruby.

Which brings us to another important topic, and one I shall try to address very carefully; the conspiracy theories about Kennedy's assassination. All three of the other assassinations were fairly clear cut affairs, with the motives and accomplices (in the case of John Wilkes Booth) being rounded up shortly thereafter. In the case of the Kennedy assassination, however, conspiracy theories relating to the assassination and the motives behind it abounded. The extent of belief in these theories varies from poll to poll and across time periods, but a sizable portion- probably a majority- do not believe the officially accepted story. This has, and indeed, continues to drive public interest in the Kennedy assassination.

Finally, Kennedy, like Lincoln, is a famous historical figure. While his assassination does somewhat overshadow the rest of his presidency, he was nonetheless a popular and influential figure both in his time and since. Kennedy took full advantage of the preponderance of television and radio; he gave the first televised presidential address in 1961. The Rasmussen poll I mentioned earlier ranked him sixth, with a net favorability rating of +67, the only post-WWII president to have a score north of +60. He was a generally well-liked president, and by the time of his assassination, it had been over 60 years since a President of the United States had been murdered. It would have been a deeply shocking and traumatizing event in a decade filled with them; in the next decade, there would be several other high profile political murders, like that of King, X, or Kennedy's own brother, Robert. Moreover, it would have been the first time that the modern news media would have reported on these.

I've tried to avoid any speculation here, and stuck as close to the facts as I could, but there just probably isn't a single overarching reason why the JFK assassination is so famous. It's a combination of the birth of the modern media cycle, of conspiracy theories about his death, of Kennedy's own personal popularity, and no doubt of a dozen other smaller factors as well, and all of these are deeply intertwined.