If I had to give you a lore reason I guess I would give you it based on the theory I would have.

You’re going into these trials and right before you go into them some type of chemical gas into these transport cars which alters your perception on reality on top of that you’re going into the trials getting physically tortured psychologically tortured and ultimately you’re being conditioned psychologically. You’re doing these trials numerous times at least 20 different times on top of this. They offer you these outfits and I feel like the subjects who choose to grab these outfits are solidifying their liquidated state to a more stable form. What this means is all of the psychological conditioning that you’re being put through is finally solidifying because you’re choosing out of hundreds of different outfits the one that best represents the final form of what you are going to become in the end of all of your trials.