As others have already mentioned football’s accessibility is a huge factor in its popularity. It is an extremely democratic sport in that you don’t necessarily need to be a 6,4″ muscular person to play it well (Messi for example). It is easy to enjoy at novice level, can be played alone or with two, 10, 22 people etc, it can be played with minimal equipment and on a variety of surfaces.
Football is a very expressive sport, different players and teams can all have their own interpretations. There is a huge variety of goals, passes, moves and just when you think you’ve seen it all a player will come up with something new. It is also very spontaneous and free flowing in comparison with other sports.