Why don't people believe in the Immaculate Conception?

Answer 1

Most Protestants believe that Mary was freed from Original Sin at the same time as everyone else. Catholics say that she was conceived free from Original Sin by the future grace that was obtained by the redemption. Catholics don't believe that Jesus would have spent his first nine months in the body of someone who was under the power of the evil, especially since people are forbidden by The Bible to receive Communion while in the state of serious sin.

Answer 2

From what I understand the Doctrine of the Immaculate Conception says that Mary was miraculously born without the Original Sin that all humans inherit from Adam and Eve.

As to why people believe it: they usually have been taught it is a doctrine of the church and must be believed to be a member of the church. That is a large part of why most church doctrines are believed.