Why were King Ferdinand and Queen Isabella called the Catholic monarchs?

The term was often used by popes to adress sovereigns who had favoured their policies.

Not only were the King and Queen of Spain referred to by the pope as "Catholiques", but the King of France was conferred the title of "Très Chretien" (Very Christian); and Henry VIII Tudor (would you believe it) was named "Fidei Defensor" (Defender of the Faith) by pope Leo X. So was James V of Scotland after him. The practice was common. Russia was "Holy" and France "The first Daughter of the Church". Some Sovereigns flew the attribute like a banner (Elisabeth II still claims Fidei Defensor as one of her titles), others ignored it as a whim of a condescending pope.

People think that the title was officially attributed to them because Ferdinand and Isabel expelled the Moors from their last foothold in Spain. But this is not so.