Requiring it to be believed for salvation contradicts the scripture.
Well, for one thing the Church does not say what will happen if one does not believe in the Immaculate Conception. We do not have a ruleset for who will and who will not be saved, because we are saved in great part by our works, and not by faith alone. However, the doctrine in question is compatible with scripture because the angel calls Mary "full of grace", "κεχαριτωμενη" (Luke 1:28), which is a past perfect passive tense of "to give grace"; she is one who had been filled with grace by the time the angel speaks. When that point was that she had been filled with grace we don't know but it is a reasonable conjecture that it occurred at the only other significant moment of her life theretofore: her conception. Hence the doctrine.