Like Pipkin42 said, it's not surprising that Kennedy is remembered well, and as you pointed out, it's not surprising that Lincoln is remembered.

Garfield was president for a very short period of time before he was shot. He was thus not able to accomplish much at the time. He also was president at a time when presidents were relatively weak. Johnson was strongly limited by the radical republicans and Congress, Grant's administration was plagued by controversy and corruption, Hayes' presidency began with the premature ending of reconstruction, and political bosses reached their peak of power. Even following Garfield's presidency, the office was perceived as relatively weak until McKinley and Roosevelt. A good book on Garfield that notes this information is Destiny of the Republic by Candice Millard.

I think your question can accurately be rephrased as "Why has McKinley's assassination been forgotten by the masses?" And I'm not sure how to answer that. McKinley was, in my opinion, a pretty significant president. He started and finished the Spanish American War, elevated the gold and silver standards, and was a standard-bearer of Republican politics at the time.

I think the fact that he was followed by Teddy Roosevelt is what answers your question. Much of what McKinley stood for (e.g. conservatism, low regulation business) was opposed by Roosevelt. The reforms Roosevelt instituted were counter to a lot of what McKinley tried to accomplish. Even after Roosevelt, you had Taft and Wilson who were reformers in their own right, to various degrees. The point is McKinley was sort of the last of the "old guard," the final gilded age president and the last of his generation.

So essentially, (1) Lincoln is remembered for his legacy, (2) Garfield is forgotten because of his short tenure and the status of the office at the time, (3) McKinley is forgotten because his legacy was cut short, and (4) Kennedy is remembered because of the recency and his cultural significance.

Well, I don't think you'll find many historians of the United States who forget about Garfield or McKinley, so perhaps this is the wrong crowd to bring that assumption to...

That said, Kennedy was more-or-less the first television President. He was (or at least presented as) young, handsome, and glamorous. He also had a glamorous wife, charismatic brothers, and the trappings of one of America's wealthiest political families. Further, there are still many people alive today who remember Kennedy's death, helping to propagate it and him in cultural memory.

Finally, there is a robust industry of conspiracy theories surrounding Kennedy's death, in part because his assassin was himself assassinated under bizarre circumstances while in police custody, and in part because amateur film and still photographs of the event lend themselves to armchair analysis. There is no direct evidence of the other assassinations, and those assassins were duly convicted in courts of law, leaving less room for conspiracy-mongering.