[W]hy is it that, whereas the priest asks them to pray for so many different things, the faithful in fact ask for one thing only–mercy? Why is this the sole cry they send forth to God?
In the first place, as we have already said, it is because this prayer implies both gratitude and confession. Secondly, to beg God’s mercy is to ask for his kingdom, that kingdom which Christ promised to give to those who seek it, assuring them that all things else of which they have need will be added unto them [Matt. 6.33]. Because of this, this prayer is sufficient for the faithful, since its application is general.
Nicholas Cabasilas, Commentary on the Divine Liturgy, (SVS Press, 1960/2002), p. 47