What caused the game of soccer to be popular?
When it comes to human tastes, it is not always possible to determine why things are or are not popular. Therefore, we can only give educated conjectures as to why soccer is popular.
The most likely reason for its popularity is that it is a relatively cheap game to play. There is very little equipment needed and it can be played almost anywhere. This helped to make it popular in times and places where people were poor, had little access to specialized playing fields, and could not afford equipment.
Of course, this does not explain, for example, why soccer became more popular than rugby, which also needs little equipment and can be played anywhere. This may be due to the association of rugby with the upper classes of Britain. It may be due to the fact that soccer is less violent. We cannot know these things for sure because human tastes are inherently impossible to explain in an objective way.