First, he is the band's bassist. M. Shadows is the lead vocalist for the band. His vocal style is characterized by its powerful, aggressive tone and often incorporates screams and growls. Christ, on the other hand, provides backing vocals and occasionally sings lead on certain songs.
Second, Christ's main focus is on playing the bass. He is known for his technical skill and proficiency on the instrument, and he often incorporates complex bass lines and solos into the band's music.
Finally, M. Shadows is the primary songwriter for the band. He writes the majority of the lyrics and music, and he has a distinct vocal style that is closely associated with the band's sound.
In addition to these reasons, it's also worth noting that Christ is not the only member of Avenged Sevenfold who contributes to the band's vocals. All of the band members provide backing vocals, and occasionally one of the other members will sing lead on a song.