I recently saw a picture of Pope Francis seated near the back of the chapel instead of up front in the special chair for the Celebrant. I am moved by this humility, and wonder why must the priest sit up front during Mass.

To clarify, Pope Francis was seated in the back of the chapel prior to the sacred liturgy. Once the liturgy began he vested and moved forward to the celebrant’s chair. 
 
The celebrant of the mass sits up front in virtue of the fact that he acts in persona Christi. Hence, the celebrant’s chair, and he, being seated in a prominent, visible place in the sanctuary, is not honoring of “Fr. Joe Smith” the man, but, rather, of Jesus Christ who acts in and through the priest, who is configured to Jesus by holy orders. 
 
In this sense, through the Sacrament of Holy Orders, the priest in the liturgy is a kind of sacrament of the presence of Christ. It is Christ who is honored, and has a prominent seat. Jesus Christ is the true celebrant and high priest of every liturgy.