I imagine starting Cade Mays has everything to do with him already being under contract next season and Christensen not.
If they feel like the performance level is similar, they probably want to see if Mays can play well enough down the stretch to win the starting job going into next season.
Plus, yeah, Brady Christensen is an excellent depth player given he can play literally every position on the OL at a solid level. Personally, I'd like to see us extend him regardless just for that alone.
He might legitimately be the most versatile OL in the league.