I don't think it's going to be too much of a generalization to say that in every show from 1974, Johnny or somebody mentions Watergate. Was it talked about on every Tonight Show from that year? Was it talked about everyday on other entertainment and news programs?
It was THE national obsession of its time. It was bigger than the O.J. Simpson murder trial, bigger than the Donald Trump "Guess What He Said Today" Follies of today. The Watergate trials were televised just about every day, and millions of people tuned in regularly to watch a bunch of politicians squirm and repeat "to the best of my recollection" and "I do not recall" over and over again with the obsession of soap-opera fans. This was before cable and reality TV and Oprah, after all. Most of the people involved in the trials wrote books about it; the chairman of the House investigation committee, Senator Sam Ervin, who sounded very much like the Southern rooster character Foghorn Leghorn from the Looney Tunes cartoons, became such a media darling that he recorded an album of songs. It was a huge deal to a lot of people, even those who weren't particularly paying close attention. And, of course, ever since, just about every scandal, political or otherwise, has the suffix "-gate" attached to it, so it even became an instantly understandable part of the English language.
Kind of funny that Ervin was compared to Foghorn Leghorn being that Foghorn Leghorn was a parody of an old-time radio character named Senator Claghorn who himself was a parody of Southern politicians. All comes full circle.