I have known many former Catholics, and here are some of the reasons they have given:
1. **Marriage outside the church**. This can mean he/she married a Protestant and went to his/her church. This also can mean that they were divorced, did not get an annulment, married again, and felt ostracized since they no longer could take communion. Etc. These folks seldom raise their children in a Catholic church, probably in no church.
2. **Moral issues**. The most common one is artificial birth control. Most Catholic couples, once they have the size family they feel they can adequately provide for, use artificial birth control. Maybe they are scolded by a priest at confession. Maybe they simply decide that if the church is wrong on that key issue (in their minds) it no longer can be seen as the infallible church when it comes to faith and morals, undermining for them its authority.
3. **Changes in the church**. Some of the older Catholics left because they disliked some of the changes, from the mass in English and folk masses to meat is okay on Friday and you don't need to fast more than an hour before communion. These changes may not have seemed important to many, but to some these tended to make them question the mystery and the authority of the church.
5. **Doctrines**. Many Catholics went to liberal colleges - for example - and there began to doubt such basic doctrines as transubstantiation, levitation and other miracles by saints, the Virgin Birth and Immaculate Conception, many stories found in the Bible, etc. These tended to drift toward liberal Protestant groups or to forsake Christianity entirely.
6. **Evangelical zeal.** Some Catholics have been attracted over the years by TV ministries, ranging from Bob Schuller, Joel Osteen and Billy Graham to some of the extreme fundamentalists who may have a gift for dynamic, emotional preaching. These services simply seem more exciting than Mass in many Catholic churches.
7. **Clergy scandal**. Catholics used to have a reverence for priests, but this has been severely undercut by sexual and financial scandals involving priests. Once this reverence is compromised, leaving the church may follow. There is perhaps even greater anger at priests high in the hierarchy who tried to cover up these clergy crimes.
8. **Media influence.** The media in much of the USA is unfriendly to religion, especially to Christianity and maybe most likely to Catholicism. This has influenced many young people, many of whom find religion irrelevant. Some find the 'rock concert' atmosphere in some evangelical churches appealing.
9. **The decline in Catholic schools**. Many fewer children attend Catholic school nowadays, so they become less influenced by the church. It is common today for a Catholic youth to be confirmed - and then, bye, bye. Peer pressure is likely to work against religious devotion. Later hey may come to back to be married in the church and baptize their children, but for many by then religion has become very secondary in their lives.
10. **Break-up of families**. Divorce, single parents, and other such factors is not conducive to church involvement. In many divorces, the children are with one parent this weekend, and that parent the next, often interfering with church attendance, especially if either parent is Protestant or unchurched entirely.