Because it contradicts a MAJOR doctrine central to and repeated over and over in the Bible (which is God's Word)...and God does not lie.
That's like saying God is going to revoke Heaven tomorrow for all believers. That would be against the promises of eternity with him for His children and would make God out to be a liar.
How does it contradict THIS Scripture:
"Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ, just as He chose us in Him before the foundation of the world, that we would be holy and blameless before Him." (Ephesians 1:3-4)
That sure fits Mary to a T.