Why does the JFK assaination resonate so much?

In the long term stable democratic countries of the world, assassinations of major political figures have not been common.  

Here is the list of assassinations of political leaders in stable democracies in the last 100 years
1932 Paul Doumer French President
1963 John Kennedy
1986 Olaf Palme, Swedish Prime Minister

The Kennedy assassination resonates because it is a very rare event.

Even in less stable democracies there have been few assassinations of political leaders:
1980 Francisco Sá Carneiro, Prime Minister Portugal
1984 Indira Gandhi Indian Prime Minister
1991 Rajiv Gandhi Indian Prime Minister
1995 Yitzak Rabin Israeli Prime Minister
2003 Zoran Đinđić, Prime Minister of Serbia

In the 1970s we did see political killings in democracies such as by the FLQ in Canada, the Red Brigades in Italy and Baader-Meinhof in Germany.  Though only Aldo Moro, a former Italian Prime Minister, was a political leader

UK politicians suffered assassinations by the IRA in the 70s and 80s, the most prominent being Louis Mountbatten and Airey Neave both in 1979

Italy suffered from a number of judges being assassinated by the Mafia from the late 1970s to the early 1990s.

Assassination is simply not something that really happens in the sort of open societies that stable democracies are.   It makes the killing of John Kennedy stand because it is so very rare.   Based on the last generation, it would seem very unlikely that we will see any major political leader assassinated in a democracy anytime soon.