According to the Catholic historian Thomas Cahill in his 1995 book
How the Irish Saved Civilization:
The early Church that grew out of Rome and the Middle East developed the Sacrament of Reconciliation quite differently from what we have today. Confession of sins was public and penance was quite severe, with absolution and public penance being only a one-shot deal type of situation. For example, if one committed adultery, then he or she would only be allowed the one public instance of penance, and no more.
However, in Ireland, Catholciism developed independently from Rome for about 200 years due to the collapse of communication between the regions as a result of the Fall of the Roman Empire. As such, frequent, private Confession was introduced by the Irish, and this practice, which is more in line with the unending mercy of God as taught by Jesus, eventually became the norm.
Addendum: I read in an older thread here on CAF that, according to other sources, there was also a certain form of private confession in Rome, but it is difficult to ascertain its actual role in that society. For example, the one-shot deal idea of penance may only have been applicable to large, public sins, but more private matters may have been dealt with more as in today’s Sacrament. I can only speak to what I read in Cahill’s book. Either way, the Church has continued Christ’s mandate to His Apostles to forgive sins.
May God bless you all!