“Nobody remembers the second man to walk on the moon” (or “No one remembers number two,” in various forms) refers to the moon walk on July 20, 1969. Both Neil Armstrong and Buzz Aldrin were on the same mission, but Armstrong got great credit for stepping on the moon seconds before Aldrin. Similar statements have used by seconds on a political ticket (such as vice presidents); the saying goes that no one remembers vice presidents, especially losing vice presidents.
 
“Nobody remembers the second man on the moon” has been cited in print since at least 1984.
   
 
Wikipedia: Buzz Aldrin
Buzz Aldrin (born Edwin Eugene Aldrin, Jr.; January 20, 1930) is an American mechanical engineer, retired United States Air Force pilot and astronaut who was the Lunar Module pilot on Apollo 11, the first manned lunar landing in history. On July 20, 1969, he was the second human being to set foot on the Moon, following mission commander Neil Armstrong.
     
Google News Archive
5 August 1982, Spartanburg (SC) Herald, “Grant Shocks Establishment” by Woody White, pg. D1, col. 1: 
White shoes were the rage when only Joe Namath could wear them. Nobody remembers the second and third and fourth Joes to cross the barrier.
     
22 April 1984, Oregonian (Portland, OR), Northwest Magazine, pg. 10, col. 1:
“I guess it’s like all things with the American public,” Jerstad adds. “The accolades go to the one who’s first, and that’s it. Nobody remembers the second man on the moon or the second man to run the sub-4-minute mile.”
 
24 December 1986, Eugene (OR) Register-Guard, “Voyager inspired dreamers,” pg. 10A, col. 1:
But no one remembers the second person to fly solo across the Atlantic or the second pilot to fly faster than the speed of sound.
 
3 March 1998, St. Peterburg (FL) Times, “Veterans Committee to vote on Hall of Fame” by John Romano, pg. 5C:
“Nobody remembers the second man to walk on the moon, but the journey was just as long for him,” said American League senior vice president Phyllis Merhige.
 
10 July 1988, Augusta (GA) Chronicle, “McKinney moves up to the gold” (Associated Press), pg. 12B, col. 5:
“I got enough silver,” McKinney said. “I want the gold, no more silver. No one remembers the second place finisher.”
(Kennedy McKinney, a boxer—ed.)
   
Google News Archive
1 August 1999, Eugene (OR) Register-Guard, “Seemingly small gain really huge” by Charles A. Jaffe (Boston Globe), Your Money, pg. C1, col. 1:   
Just as no one remembers the second man on the moon, vice presidential candidates who lose or the Boston Marathon runner-up, a milestone birthday for the second fund is no big deal.
 
15 December 2000, Boston (MA) Herald, “Gore shows his human side” by Beverly Beckham, pg. 39:
We hitch our wagon to winners. And to firs. Nobody remembers the second man to walk on the moon. Or the second man to cross the Atlantic. Or the runnersup in
 
Google Books
You’re Hired:
How to succeed in business and life
By Bill Rancic and Daniel Paisner
New York, NY: HarperBusiness
2004
Pg. 76:
About the only negative we could pinpoint was that the Cigars Around the World concept was so simple it could easily be pirated by a competitor, but we figured we’d have a running start and reminded ourselves that nobody remembers the second team of astronauts to walk on the moon. It’s the pioneers who get all the credit—and, we hoped, the bulk of the business.
 
6 February 2007, Atlanta (GA) Journal-Constitution, “Colts weather storm despite first appearances”, pg. C3:
Nobody remembers the second man to walk on the moon. Plus, most US presidents between George Washington and Abraham Lincoln are a blur.
   
Faceoff.com
All Nfld. cheering Cleary
DETROIT - Alex Faulkner is pleased to hear a voice from Windsor on the other end of the telephone.
By The Windsor Star May 26, 2008
(...)
“Nobody remembers the second guy to walk on the moon,” said Faulkner, the first Newfoundland-born player to perform in the Stanley Cup final for the Detroit Red Wings—in both the 1963 and 1964 final series against the Toronto Maple Leafs.