My experience is that many left the Church after Vatican II because many priests and laity became liberal or latitudinarian in their theology. Many Catholics turned to the fundamentalists because they at least knew who they were. Many others just stopped going to mass after the liturgical reforms which threw away forms of worship they had grown up with.
My experience is, among my siblings and cousins, is that they left because they were horribly mistreated and abused by vicious nuns in Catholic schools. Some have joined other churches, some are atheists and agnostics, and a few were CINOs long enough for their daughters to have a wedding pageant.
I know only **one** person who is truly a faithful and practicing Catholic. A very sweet man whom I respect and admire. He is a friend and did not attend a Catholic school.