First I think they may have a problem with and one or more of Church policies, birth control policies of the Church, the Church’s pro-life versus the popular cultures pro choice stance, being required to go to mass every week, obeying Church dogmas, etc
Second, I think the see so much criticism about the Church and they start to believe some of this non-sense. They see that the Church does not emphasize scriptures as much as protestant sects.
They are impressed with Bible quoting ministers and members, or they are unable to defend their belief because they are not aware of the scriptural backing or historical background.
They start to believe the Bible only half truths, and ignore the traditions and Chruch Councils that developed Church doctrine.
Being Catholic is not easy, we are required to do so much more, fasting, praying, abstinince, charity. It is much simpler to just say “I believe in Jesus”, and forget about the alms giving and good works.
You can save a lot of money by not giving to the Church or the poor. (you may end up in H***, but that does seem to matter to them.
And last, the temptations of the world are too much for them to give up. The pursuit of money, power and pleasure (and maybe even drugs or alchohol) overtakes their sense of duty to God and humankind.
There must be 50 ways (probably a whole lot more) or reasons (excuses) to leave the Church, none or them good or appropriate and none them can ever override the main reason for staying. Jesus and His Chruch holds the keys to eternal life, where can we turn if we leave Him or His Church.
wc