Answer:
Spain does not have the most beautiful beaches in the world. The most beautiful beaches can be found on Caribbean or Pacific islands. Spanish beaches are nice, I spent 10 years laying on them, but what makes them so popular is their proximity to the masses of Europe. Most of the Mediterranean beaches benefit from this proximity and the fantastic Mediterranean weather.
What they do not have is the crystal clear blue water of the Pacific and Caribbean beaches. That water is so clear in some places that a ships anchor is clearly visible in 60+ feet of water sitting on a weed free sandy bottom. And there are not so many people.