There can’t be just one reason because there are so many reasons. Grace. Unity. Hope. Love. Faith. These are the big reasons. One may actually find these attributes in other Christianities, in Judaism, other religions and probably even in the secular culture.
I think the reason for Catholic in particular is Apostolic Succession. It’s something that once seen and grasped cannot be unseen. I think many Christians who aren’t Catholic don’t see it. Perhaps they are even prevented from seeing it. Thus they cannot be judged against it.
In my case I arrived to the Church by way of exploring my Jewish roots. My research into spirituality and history convinced me that Temple Judaism had the strongest claim on truth of any religion. Admittedly I got on the path to become Catholic before I had a lucid understanding of succession. I superficially understood that Catholicism was the “fulfillment” of Judaism but didn’t really understand why. Only now that I’ve gone through the door (confirmation) has it been revealed to me, via the research of Catholic theologians, how Catholicism is the fulfillment of Temple Judaism.
The cultic religion of the ancient Jews was God’s plan to incubate the Messiah. God’s secret, even from the Jews, was that His person of the son would incarnate as a man to redeem humanity. True, divinely commanded cultic temple ceremonial praxis, once exclusive to Jews and God-fearers, was to be expanded to the entire world by way of the real body of God, the true temple, in the Eucharist.
Jesus was not a priest in the order of Levi. He was a priest of the universal order of Melchizedek. It is my current understanding that all Israelites are born into this order. Certainly his royal ancestor David is called a priest (forever!) in the order of Melchizedek in scripture.
Jesus dissolved the Levitical priesthood because it had fulfilled its purpose. It had successfully prepared the world for the Messiah. Jesus ordained his apostles as the new Priesthood. The Apostles were the seed of the new universal church. The authority to bind and loose on heaven and earth, previously given to the Temple Priests, was transferred to the new priesthood of the Apostles.
When I understood that the Catholic church is the preserved succession of the priesthood of God incarnate by ordination it reinforced why I had to become Catholic.
As in the prior temple, corruption can be a problem in any institution. However, human failings do not undermine a priesthood’s legitimacy. The Church’s legitimacy traces a direct line from Jesus who is God.
I can understand the difficulty Jews may have in joining the Church as institution without the understanding of succession. The perception of the institution, and certainly some Catholic people, is that they were not kind to our ancestors. I take comfort knowing that Jesus the founder and the apostles were Jewish. I found the bodily present Messiah, the everlasting Temple.
Once you see it, whether you are a Jew or a Gentile, you cannot unsee it. Importantly, you are accountable because of that sight.
