Largely to do with different theologies.
In Protestant Theology, Grace is God's gift of forgiveness to the unworthy, in Catholic theology, one earns Grace by seeking to bring one's self closer to God.
This leads to two divergent ideas of forgiveness (I'm going to oversimplify to make a point, rest assured it's a bit more complicated than this).
In Protestantism, since Grace is unmerited, confession is very often "Sorry God" "THAT'S OK, I FORGIVE YOU".
In Catholicism, since Grace is merited, forgiveness is a process of penitence. The confessor does not "mediate" between the individual and God, but acts as a spiritual guide who seeks to open new ways of understanding their relationship with God to the individual, and through this process to guide them towards a more morally worthy life.