Well, the snug-fitting bit (the bib tops) are meant to go against your skin and provide a rub-free contact. The looser part (the jersey) goes on top. By putting the jersey under the bib, you are creating a rub point around the waist (ie, at the point where the bib goes over the jersey or where the jersey tucks into the bib) and also creating a friction point in case the jersey ever bunches up under the bib tops.

Plus the other reasons already mentioned.

V.