Why was the Renaissance age called "The Golden Age of Acappella Singing?"

The Renaissance took place in the fourteen through sixteenth centuries. It was a time of incredible development in the arts. We see great advancements in painting, sculpting, and architecture, to name a few things. In light of this, it should not be surprising that there was also a great development in music. For instance, we see that the origins of opera began here. Catherina de Medici brought this new music from Florence to France. Also it was a time of great growth in church music and it further developed the gregorian chant. It is perhaps for this reason that the renaisance was called the "golden age of Acapella." See link for a brief history.