I've felt for a while that K was a bubble starter. I think that he does his job to the best of his ability, takes coaching well, and can execute against mid and lower tier tallent. Do I feel that Vollmer is a better player? Yep. Do I think Vollmer is going to be a top three NFL tackle in a few years? Yep, that too.
In a pass heavy offense, you need a player with good footwork, wingspan, and hand technique on each side of the line. Nick is good, and serviceable at these things. Vollmer is exceptional. It also doesn't hurt that Vollmer is lethal run blocking and accelerates like an end in the screen game.
Whatever his injury or issue Kaczur is currently having, it is preventing him from executing. He's over reaching on speed stuff, and is getting stood up on the power stuff. It's only natural to have your best players on the field, and that means that Vollmer needs to start on the right side.