John Calvin Commentary Matthew 23:10

"Neither be ye called masters: for one is your master, [even] the Christ." — Matthew 23:10 (ASV)

For one is your Master, even Christ. He repeats a second time the former statement about Christ’s office as Master, in order to inform us of the lawful order: God alone is to rule over us and possess the power and authority of a Father, and Christ is to subject all to His doctrine and have them as disciples. As it is elsewhere said, Christ is the only head of the whole Church (Ephesians 1:22).

Because the whole body ought to be subject to Him and obey Him.