What, precisely, are you asking? Are you unsure why the 1970, 1974, and 1978 FIFA Football World Cups were major sporting events without the United States?

What was a major sporting event that USA enjoyed?

American NFL and college football (American style football, not "soccer"), along with Major League Baseball (MLB) as well as the National Basketball Association (NBA), have always been the real big three "major sporting events" that the Yanks in America have really got behind and enjoyed the heck out of. This is certainly the case today, as was also certainly the case back in the 1970's as well.

Major League Baseball has long been known and appreciated by the moniker of being the "great American pastime" in fact, and NFL and college football consistently draw massive TV viewer ratings throughout the season (and especially during the "SuperBowl" final game) and, along with the NBA, enjoy a huge fanbase around the nation. In fact, if you look at the list of various sports leagues rankings, as categorized by revenue generated, you see the NFL, MLB, and NBA seem to hold the top spots. (I realize that wikipedia is usually frowned upon here on this subreddit as a source, but I believe the numbers shown here on this wiki page are pretty accurate, so decided to include it here).

Heck, even in smaller towns in America, high school "American style football" can get downright crazy in terms of local support. Some high school teams - students below the age of 18 playing sports for their high school - in the state of Texas for example have multi-million dollar stadiums they play in, and can draw (in the cases of fields like Memorial Stadium in Mesquite, Texas and Toyota Stadium in Frisco, Texas for example) over 20,000 fans to a game! They LOVE their football in Texas let me tell ya!

Soccer had not really caught on in America like it has elsewhere around the globe, though the "beautiful game" definitely has seen a big growth in support in the US of A in the past couple decades. Many arguments as to why soccer is not as popular in America tend to focus on the pace of the game and the perceived limited scope of action, as soccer tends to have a lot of the 90-minute play period with not much going on in terms of goal-scoring opportunities, accentuated by very brief flurries of action and intensity. Also, I think it is fair to say that many Americans are put off by some of the rules and play style in soccer, with the offsides rule being a point of contention for example, as well as some of the "diving" or embellishment by players faking injury or fouls that many Americans seem, from my experience talking about this at least, to consider "unmanly" and very unsportsmanlike behaviour.

As to why FIFA soccer was a major sporting event in the 1970s globally, even though the USA did not really participate much in it, it needs to be appreciated that soccer is a VERY cheap game to play and participate in at the youth level, so countless underdeveloped countries can use it as the sport of choice for their young populations with minimal overhead cost to begin building talent. All you really need is a ball to kick and an open field to play that game on, whereas with the NFL-style football you need expensive protective equipment to properly play, and MLB baseball (or even "sandlot" baseball played by kids) tends to require costly bats, baseball gloves, and multiple balls on hand to play effectively. Even NBA-style basketball requires a properly-laid out court with hoops and backboards to be played properly. Due to America's wealth as a nation, they were able to afford decent "playground" equipment for football, baseball, and basketball at schools and parks around the country, allowing kids at the youth level to get into those sports, and thus get very good at them. I think that plays a big role in why those sports - which are definitely more costly to participate in than soccer is - have enjoyed such success in a relatively wealthy nation like America, whereas soccer, being so cheap (relatively speaking) to participate in at a young age, failed to pick up a lot of steam until more recently.