The Bible teaches that all humans are sinners. I don't think the apostles would have passed on by word of mouth the doctrine of the so-called immaculate conception since it so dramatically contradicts their writings in the New Testament which emphasize over and over that all men sin. Why would anyone believe anything they had to say if they contradicted themselves in that way? This is why Protestants can't believe the doctrine...not so much that it is based on supposed verbal tradition, but that it is overtly against the apostles' Biblical writings.
For example, why would John in Ephesus write in his epistle in 1 John 1:10 that "if we claim we have not sinned we make him out to be a liar..." if there was actually one who was living or had lived among them in Ephesus (Mary) who supposedly had not sinned?
Or why would Paul write, "For all have sinned and fall short of the glory of God" if he knew Mary was not a sinner? In order to make that not a falsehood he would have had to have added "for all have sinned except Mary". Paul especially says over and over and over again that all of humankind except Christ is in a lost condition because of sin. I take him at his word.