"It's all about the money." You've probably heard critics of FIFA's proposed biennial World Cup make that very point, and guess what? It largely is. And, from FIFA's perspective, there's nothing wrong with that. Their core mission is literally "develop the game, touch the world, build a better future."
All those things take money or, at the very least, are a heck of a lot easier to do with money. FIFA funds -- mostly the Forward Programme -- is pretty much the only source of revenue for a majority of federations around the world, and because the World Cup is basically FIFA's only source of revenue to funds its member federations, why wouldn't they want to have twice as many World Cups per four-year cycle?