Why don't people believe in the Immaculate Conception?

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.