Why does a priest say "The Lord Be With You” and a bishop say "Peace Be With You" as a liturgical greeting?

One [option](https://christianity.stackexchange.com/questions/23290/why-are-there-differences-in-the-greeting-of-the-mass?rq=1) for the liturgical greeting at the beginning of the Mass is: >Priest: The Lord be with you. > >[or Bishop: Peace be with you.] > >All: And with your spirit. The phrase "The Lord be with you / And with your spirit." is used in other parts of the Mass too without that difference. - Where does this difference between bishop and "ordinary" priest come from? - Are there theological and/or historical reasons?