The simple answer is the sport’s global popularity. While football and basketball may be more popular than soccer in the United States, soccer is the most popular sport on a global scale. We can see this by checking simple metrics, such as the follower counts of major players in these sports.
For example, it’s fair to say that LeBron James is the biggest basketball star in the world right now. He currently has about 70 million Instagram followers. Compare this to Cristiano Ronaldo, who is soccer’s biggest star. He has over 230 million followers. The point is that the audience for soccer cards is larger than the audience for cards made for other sports. As such, sellers should find it easier to make a profit from their soccer cards.