The answers are as numerous as there are people. But here’s just a few big oness:
1. Suppression of Christianity
Many people live in countries where Christianity is persecuted and even outlawed. Think Islamic countries and Communist ones in particular. You can count on zero hands the number of Christian churches in, say, Saudi Arabia. When the penalty for becoming a Christian is death, fewer people become Christians; fewer can even give it a fair shake, presuming they can obtain accurate information about the Faith.
2. Sin
Sin is a reality in the world. It makes us stupid. It can dull or even kill our conscience. It clouds our judgment; it wounded our very nature and darkened our intellects (though not completely!). As such, we have trouble exercising our God-given rational powers correctly.
Also, sin leads us to preferring the glitter of evil to the (always more difficult) narrow road to truth. This is why we renounce the devil and the allure of evil when we renew our baptismal promises.
3. Bias Growing Up
Even people who are Christians often grow up in a home or church that are biased against Catholicism. Without even being aware of it, these persons are indoctrinated in anti-Catholic ways that are very difficult to overcome. Francis Beckwith had an insightful reflection on this subject which he termed “plausibility structures.”
4. Toxic Culture
Cultural propaganda can be toxic to Catholicism. In our country, scientism and moral relativism have taken deep root in the psyche of most Americans and are pervasive in schools. These erroneous ideologies are antithetical to Catholicism.
5. Scandals
All these would make it hard enough to become Catholic. But add on top of them the scandalous actions of Catholics throughout history, from ancient times to the priestly sexual abuse scandal recently. Terrible. It takes someone with a strong faith to be able to see past these evils to the truth of the Church herself.
That’s what comes off the top of my head. What other obstacles do people face to seeing the truth of the Catholic Church?