Why was the church upset about the elaborate polyphonic setting of the Mass?

Polyphonic music is a type of music that consists of two or more lines of independent melody, as opposed to having a single voice or a single melodic voice.

The Catholic Church was upset about the use of polyphonic music in mass prior to the period of the Western Schism. The main reason for this was that many people believed that, by using polyphony, secular music was being allowed to mix with sacred music. This removed the solemnity that a mass demanded. Moreover, such type of music also made it more difficult to understand the words of the mass. However, attitudes varied among different proponents and different regions.