Why did early polyphonic music require trained solo singers?

Early polyphonic music required trained solo singers primarily because the complexity of multipart music demanded a high level of interpretive skill that was not expected of the average member of a plainchant choir.1

Several factors contributed to this requirement:

• Notation Limitations: Early forms of musical notation lacked the precision necessary for choral performance of polyphony. Understanding the subtleties of mensural (precisely measured) music was initially the prerogative of a small group of initiates rather than the general schola.2
• Technical Complexity: As polyphony emerged, its multipart structure required singers to maintain independent melodic lines synchronously, a task that necessitated specialized training.13
• Evolution of Skill: Over time, the teaching of musical theory spread, and singers became better equipped. By the 15th and 16th centuries, the rise of great choral establishments led to the systematic training of singers—including choirboys—who were considered soloists in their own right.2

Eventually, as notation became more accurate and choral establishments grew in opulence, the exclusive reliance on soloists for polyphony was modified, allowing for the development of choral performance as it is understood today.21