why is 1972 title games called games of century？

The Game of the Century does not refer to games during the 1972 Fischer Spassky title match.

The Game of the Century was a game that Fischer played against Donald Byrne is 1956 I believe.

It was called that in 1956.  The name stuck.

There was no panel discussion at the Millennium that overturned that decision.

Oh its informative :blush

1972 was called the match of the century.
It was beween the American challenger Fischer and the World Champion Boris Spassky of the Soviet Union and that in an era of Cold War. So it got much international media coverage.

Prior to this match all post World War II World Champions and their Challengers had been from the Soviet Union: Botvinnik, Bronstein, Smyslov, Tal, Petrosian, Spassky so the matches had been an internal affair between opponents from the same country. Chess was a matter of national prestige in the Soviet Union. Fischer broke the hegemony.

He started 0-2 down (losing the 1st and forfeiting the 2nd) and finished the 24-game match after just 21 games with 12.5 - 8.5. Here is the most famous game
https://www.chessgames.com/perl/chessgame?gid=1008423 

     During the Cold War, many international sports events had a significant sub-context of USA vs USSR: who would win the most medals at the Olympics, the world figure-skating championships, the world gymnastics competitions, etc. There were much-heralded USSR-USA track-and-field meets. Each side had a big celebration and publicity campaign whenever they won--"proving" their system was superior. When the US hockey team beat the Soviets it was called "the miracle on ice" and is still talked about 40 years later; the same is true of the Soviet win over the US basketball team in 1972.

      There was also the "space race" to see who could achieve the most "firsts" in space travel. The Soviets were first in most achievements, including the first man into space, but the Americans won the big prize with the first man on the moon. All this was trumpeted with massive publicity about how "our side" (whichever won something) was "Number 1.

     In the larger competition, each side was seeking to gain followers in neutral countries and gain allies and international influence. They couldn't really get into a war with each other because each side had enough nuclear missiles, submarines, and bombers to utterly destroy the other in a matter of hours. so the fierce competition was sidelined into other areas.

     Into this situation came Bobby Fisher, emblematic of the "self-made" American hero who would single-handedly take on the hitherto invincible Soviet chess "machine". Hundreds of reporters covered the drama. Newspapers, radio, and TV outlets that had never mentioned chess had the match on their "front page". Many millions of people worldwide who knew nothing about chess were excited about this match. Myriads of new players everywhere took up the game. Fischer's demands and threats to withdraw added to the excitement. 

     As we know , Fischer won but then quit, and the hoopla over chess championships has never again reached such heights.

It can stay GOTC. But there should also be an official game of each decade. And more subdivisions; the most brilliant / attacking / defensive / unconventional / sacrificial / positional / best miniature etc. game of the century / decade.