Here's why:
* Common nouns are general names for people, places, things, or ideas. They are not capitalized unless they are the first word of a sentence.
* "World Cup" is not a specific, unique tournament. It's a general term for any sporting event held internationally, like the FIFA World Cup or the Rugby World Cup.
However, when referring to a specific tournament, like the "FIFA World Cup," it can be considered a proper noun, which should be capitalized.