I have never before seen "FIFA World Cup" used so much and I guess that's why. Do you enjoy watching FIA F1 or IOC Olympics? It's pushy and inconsiderate self-promotion that nobody wants to hear.
I also wonder if it's a trademark thing -- there are other world cups, but there's only one FIFA capital-W world capital-C cup. Just off the top of my head, the last decade has had cricket, rugby, basketball, and ice hockey world cups.