I am kind of an odd duck. I converted to Catholicism as an adult and was sufficiently Catechized. My problems started when a woman was excommunicated for procuring an abortion for her 9 year old daughter who was pregnant with twins. If the twins were carried to the point of viability there was less than an 10 percent chance that any of the three would survive, from what I read. But, the step-father, who spent three years raping the girl, was not excommunicated. When I heard of this, I began to doubt that this church actually represented Christ. So, I started to look into the reformation and other issues and I consistently found that the Catholic teachings in relation to these issues was not an accurate representation of what actually happened. So, I looked into the Sacraments and their institutions and discovered that Jesus did not actually institute all the Sacraments and the Catholic Church teaches that he did. This contradiction in the core teachings was enough to drive me away.
It is important to note that nothing really came from without here. Everything that drove me away came from within.
Also, I’m not going to argue if my perceptions are correct or not on this thread. The OP wanted to know why people left so that is all I will talk about on this thread. I will be glad to clarify things, but I will not defend them as that would be off topic.