It's more surprising that it's as popular as it is in the world in general. When being a spectator, the main draws of a game are the ability to tell a narrative and for excitement to build with a climax. Of the five biggest sports in the United States I'd rank them as follows.
1. American Football: Easily the best narrative of any sport. The game can be sub-divided into three separate categories mini-dramas on almost every play. The need to get 10 yards in 3 plays to continue a drive, the narrative builds as the make-or-break 3rd down approaches. Where the team is on a drive, the closer to scoring the team is, the more exciting the play, and the overall score within the game. And, of course, there are big plays that happen within the narrative that scramble the whole story.
No other sport comes close to this level of narrative brilliance, and that's why it has eclipsed the others.
2. Baseball: Surprised to see this so high? You shouldn't be. Baseball is highly underrated as a spectator sport. Like football, it has a rising narrative and multiple dramas within each pitch. The tone of the game changes based on whether the count is 0-1 or 3-1. Whether someone is on base, how many outs there are. It's more nuanced than football, so it requires a bit more knowledge, which drops it well behind the other sport.
Tied for 3rd: Basketball and Hockey:
Ironically, these are the two fastest-paced games of the five, but they both have huge deficiencies in their design.
Basketball's flaw is that a goal is worth so little. You could watch a player make the most amazing play ever, and it'll still only be worth two points. Roughly 1/50th of what's needed to win a game. You get a lot of them, but there's more of a feeling of watching whose point total can go up faster than a sense of who is really doing better. If a team wins by a typical score of 105-95. What does that make them? 10% better? A few, almost random makes and misses that seemed meaningless when it happened and that score is overturned.
Hockey might well be my favorite, but its flaw is that the scoring feels almost random. It's hard to know when a goal is going to be scored or the goalie is going to make a save. There are a decent number of goals per game, which is good, but outside of powerplays, it's hard to know when to expect them. Spectators are left to cheer when something happens instead of expecting something to happen and occasionally being blindsided by a big play. Instead it's all or nothing.
5. Soccer: Oh boy, this is bad. Take all of hockey's flaws, make the game slower, lower scoring, and toss in the dumbest rule in sports, the soccer offsides rule, which inspires bad defense so that the ref can bail them out (toss the rule out once they get to the top of the box guys), grown men crying on the ground for minutes at a time as a delaying tactic, and a weirdly all or nothing penalty system. 90 minutes and pretty much nothing can happen until it randomly does. Because the goals are spaced so far apart and come so out of the blue, there's really no edge of your seat excitement until just moments before it happens. So you're watching a long, boring game, punctuated by a few moments. It's the worst of all worlds and by far the least exciting sport to watch as a spectator.
Football and Baseball are definitely the best, but the reason they aren't so popular is simple. The chance to pick up and play. Football and Baseball takes a lot of strategy and players, so it is harder to understand for viewers.
In terms of ease it goes like this:
Tied for first: Soccer and Basketball. Can 1v1 and it is still the same or 2v2, 5v5 and etc. for a full game. Just find a field/urban landscape to put 2 goals in.
3rd: Hockey, similar to Soccer and Basketball but requires more equipment.
4th: Football, Requires more people than a 1v1 to actually play football, otherwise you just throwing it back and forth.
5th: Baseball, Similar to Football, but requires more equipment.