The reason why Super Bowl is more "popular" than world cup is the fact that maybe there are more people watching the world cup but it is aired everywhere around the world and the total combined could make up for a bigger audience however it is in every single nation and not compact in just one nation, so when you look at it Super Bowl is watched more than any country watches World Cup in just one nation.

Just to give an example, lets make up the numbers, World Cup gets 10 million viewers in England and 100 million in the world, Superbowl gets 50 million in USA, now numbers are made up of course but as you can see the difference between one nation thing and whole world thing. No company can make ads all around the world for World Cup but on Super Bowl american companies can make american ads to american people and reach more people.