I see several very clear, distinct reason for those who before called themselves Catholics to no longer do so.
The largest, single reason is that these were CINOs, Catholics in Name Only, who were only socially aligned with Catholicism. Where they had lived the church effectively ruled over them, dominating the government, the police, and all the good jobs. It even after a fashion dominated the criminal element. The ticket to anything was that a person had to call themselves a Catholic and be affiliated with a parish.
When this social omnipotence of the church ended, so did any reason the CINOs had to be anything other than nominal members.
The second biggest reason was Vatican II. To traditional, conservative Catholics, it felt like a betrayal at the top.
I got to see what happened when a church in Phoenix was again allowed to conduct a Latin Mass. Catholics who had not been to church since Vatican II *had* to return. The first two rows of pews were filled with aged invalids, some on death’s door. Standing room only in that large church, with floor fans because of the heat of so many crowded together. A few elderly people carried icons of their deceased spouses.
These were people who felt the church had deserted them. They wanted their religion back, not whatever trendy ideas were popular for the moment.
A major reason is that many choose to leave the denomination for a different Christian denomination.
BTTT!