Someone please correct me if I'm totally wrong, but my conjecture is that the everted tissue is swollen and the tissue contains a lot of water. Also, being part of the internal part of the animal it is probably quite porous
I think soaking the tissue in a solution with a high concentration of sugar (as opposed to salt, which I imagine would sting like an SOB) draws out the excess water in the tissue, therefore reducing the swelling and inflammation without drying it out. This reduction in swelling allows the tissue to slip back into its natural state (inside the snake) while the water keeps the tissue supple and undamaged during the process.
It's got something to do with water's tendency to stabilize concentrations of dissolved substance, I think. Kind of like if you put salt on a slug and it will start to dry it out.
Then again I could be way off the mark. It's a good question and I'd actually like to know the real answer to as well. However I couldn't help but take a stab at it.