Probably because the first time we did it we were an optimistic country with a good economy and a bright future in front of us?
Newsman Walter Cronkite remembers the year of Apollo 8: "The whole 1960s really culminating in 1968 were the most terrible decade, undoubtedly, of the twentieth century and very possibly our entire history, even including the decade of the Civil War. America was divided as it never had been since the Civil War and by the Vietnam War, by the civil rights fight.
"Everything seemed to come to a head in '68. There were the assassinations of two of the leaders of the more liberal causes. Bobby Kennedy, shortly after winning that election in California that probably would have put him over the top as the presidential candidate that year, and Martin Luther King, of course, in Memphis, was a terrible blow to the entire cause of civil rights. By the summer of '68 the Democratic convention turned out to be a terrible shambles of violence and counter-violence by the Chicago police... By December the country was pretty far down."