For starters, the whole planet would be on the same schedule, which means no more European clubs whining about losing players to international competition. Those continents which, for climate reasons, can't have their international tournaments in July or August can schedule them in December. If you cram most of the qualifying matches in July and August, you greatly reduce the stress of having them throughout the season. But most of all, the fans would get breaks from the game, which means they will come back hungrier for more. Right now, soccer is too much of a good thing.
Some fans get bored with what feels like a neverending season.
It's a little bit like giving ice cream to a kid. Forcefeed him a tub of French vanilla every hour on the hour and you will traumatize him for life. They have a good thing going and don't want FIFA cramping their style. You only need to see the reaction of the Premiership and the Primera Liga when UEFA asked them to reduce their top flight to 18 teams. They threw a tantrum and thumbed their noses at authority.