There are many, many reasons.
The sex abuse.
The perception of being anti-woman and anti-gay.
Other churches are āeasierā and theyāll let you have pre-martial sex, live together before youāre married, divorce, no obligation to attend services, etc. Why would you want to go to a church that makes you wait until marriage for sex, makes you go to confession, allows women to be ministers, allows ministers to be married, services are more engaging and fun, and on and on and on.
On the other side of the coin, you have fire and brimstone preachers talking about idol worship, worshiping Mary and the saints, having the wrong bible, being the whore of Babylon, etc. The Jack Chick-type protestants that see a conspiracy behind everything the Catholic church does appeals to a lot of people.
Atheism is also very appealing. You get bonus points for being a āfree thinker,ā and you just have to live up to the standards you set for yourself, which are usually pretty low. (When was the last time you saw an atheist give everything away to live in poverty to help others?) You can do whatever you want and not be held accountable. Have you ever read Mark Twainās The Mysterious Stranger? At the end, when the main character is told by an angel that there is no God, heaven, or hell, he isnāt sad, heās relieved. Who wouldnāt find that appealing?