It comes down to several reasons.

Firstly, as sad as it is to say, there are a fair number of people - particularly from the younger generation that were not alive to witness those historic Apollo missions live - who today actually have no idea that there was more than one Moon landing, and even if they do, many of them could not tell you how many occurred, let alone tell you how many men have even walked on the Moon. Even if taught it in school, the vast majority of lesson plan focus was on Apollo 11, leaving the rest of the Apollo astronauts and their monumental achievements largely forgotten by everyone except the comparatively few die-hard real lovers of manned spaceflight history.

Only 24 humans from the planet Earth have left low-Earth orbit and have made the voyage across cis-lunar space to visit the Moon, and just 12 have walked upon its surface, with all returning home safely. Most everyone knows of "Neil and Buzz" of course, and their place was cemented in the history books because they were the first to land on the surface and walk around up there. Humans tend to, first and foremost, focus on historic "firsts". Who is recognized as the second European to visit the "new world" after Christopher Columbus "discovered" it? Who was the next person to fly in an aircraft after Orville and Wilbur Wright did? Who came after Edmund Hillary and Tenzing Norgay to get to the top of Mount Everest? Who was the third man (on Apollo 12), or even who was the last man (on Apollo 17), to walk on the Moon?

Of course, many people who frequent this particular focused subreddit, which obviously attracts lovers of human history, know these answers. But if you walked down the street and polled the first 100 "average" individuals you came across, I am willing to bet that many would not have a clue of the answer to any of those queries. Many people seem to follow the "Ricky Bobby" (from the film "Talladega Nights") mantra - where "If you ain't first, you're last!" They don't care who was next in line - they are only really interested in knowing who was at the very front of the line, and that bit of info is good enough for them. It is human nature it seems for many to think that way.

Now, with the specific question you asked about the Apollo Program, you can really see how that pervasive mindset played out around the globe. The Apollo 8 mission in 1968 for example - the first mission to send humans to orbit the Moon - was absolutely HUGE news around the world at the time. Borman, Lovell, and Anders became household names, at least for a short time afterwards. Then of course the big names associated with Apollo became "Neil and Buzz" when they made the "one giant leap for mankind" in 1969. That achievement pretty much made Apollo 11 the single most popular publicized event in human history at that point, thanks to the ability for huge numbers of people to follow the drama live via the television factor.

Unfortunately though, After Apollo 11, there was a dramatic drop-off in the public's attention about the remaining missions to the Moon. This was in large part attributable to the global press, who also very quickly grew tired of continuing to put the huge effort into giving the public the same massive coverage they had provided to us all from the early days of the space race - from Mercury through to Apollo 11. America had, in many people's minds (including the press) won the "space race" with Apollo 11. Many figured, why bother continuing to cover the remaining missions to the same degree? Sadly, the idea of "been there, done that already. Who cares?" seemed to become the prevailing mindset for many in both the press and public. It was like the mindset was that the others who followed Apollo 11 just went, picked up some rocks, bounced around up there in 1/6th G, and then came home. Nothing new to see! Oh how wrong they were, but that was the way it was appreciated for many in the public.

Also, keep in mind that Apollo 11 landed on the evening of July 20th, which snagged that first landing and first moonwalk a perfect "prime time" TV spot across North America. The other Apollo missions however, due to issues dealing with ensuring proper lighting/visual conditions for the astronauts during the touchdown phase at their landing locations on the Moon, took place in the wee hours of the morning across North America. That also made those later missions much harder to publicize live, and drastically dropped the public viewing audience appeal.

NASA's public relations arm also fell off very sharply after Apollo 11 - the mission which had rightly been hyped to hell and back for many months prior. That lack of attention from the press and the lack of the massive publicity arm pushing the story, which no longer needed to stress the "who will win the space race - America or the Soviets?" question during the heart of the Cold War, really hurt the publicity of the rest of the amazing Apollo efforts after Apollo 11. The battle had been seen as won with the first landing, and at tremendous financial cost too from the taxpayers who funded the whole thing. People had spent a decade getting ready for the "one giant leap", and once it happened and Neil Armstrong and Buzz Aldrin and Mike Collins returned home safely, the hype of those other five Apollo flights sadly faded very quickly. As insane as it sounds, many people were tired of hearing about it!

Hell, even when they had the chance to show live footage of the later moonwalks with color TV cameras up there, the TV networks were instead showing re-runs of soap operas, comedy, and game shows! It took the "thrill" and drama of the near-deaths of the Apollo 13 crew to suddenly snap the public back into watching and being enthralled for a short few days, but once Lovell, Swigert, and Haise returned safely to the Earth after that spacecraft emergency, most people went right back to not caring about the remaining flights to the Moon. Sad, isn't it?