I'm back after a long absence... Happy to see that topics are still very interesting around here
I'm with Cyborg and urban here.
I think the main factors are
1) Money : playing, among others, the almost forgotten Champion's Classic proved and 'offer he couldn't refuse', and who are we to judge? Tennis players did not have a lifetime of endorsements, commenting gigs and sometimes talk-shows ahead of them as they have today...
2)History: people who were watching tennis in these days might correct me, but I feel there was less talk about history, all-time greatness, etc. Few people cared about Tilden or Doherty in the 70s, so Laver being the absolute king of his era, that might have taken away some motivation to crush records at Wimbledon or the US Open. Even if Federer is no tennis history expert (he might be, for a player, actually), there is a sense that he'll always have another feat from the past to beat, that is more prevalent in the press.
3)Context: the early Open era was a revolution that blew away a lot of tennis's conventional wisdom... This was probably the time were the Majors were the least...major! Even if others went for Newcombe or Rosewall, in late 1970, the Times of London clearly named Laver the world's top player, despite mediocre Slam results. After his 1973 Dallas success, Stan Smith stated he felt for the first time like he had surpassed the Rocket...though he had won 2 majors + a Masters crown in the past 3 years, and Laver had lost at every occasion...
So it is likely that Laver was simply adjusting to the criteria of his time. If the journalists and fans had endlessly discussed about his decline after his Wimbledon loss, as they would today, he might have prepared a lot better for the next edition...
Have a nice evening,