If we are under water for minutes or hours, our skin appears shrunk, shriveled, and pale. How does a baby's skin appears normal even after 9 months in amniotic fluid?
Wrinkling of wet skin is triggered by an increased concentration of water in the outer layers of the skin. This happens because water tends to move from areas of low dissolved substances (eg. a bucket of tap water) to areas of high dissolved substances (eg. your body).
Amniotic fluid is isotonic: that is, it has the same concentration of dissolved substances as the body. This means there's no movement of water, and hence, no wrinkled skin.
Like the other answer, in part I am sure it is literally that amniotic fluid is not water. Your hands will not wrinkle in all fluids, they do in some fluids. I also don't even know if they would absorb water that early even if exposed. You are covered in vernix at birth, and it's waxy & protective. That alone would likely act as a water barrier. You will also notice in wrinkling it's only in the hands & feet. That type of wrinkling is intended to improve traction on slippery surfaces. You don't wrinkle everywhere when submerged, only in the areas where it may benefit you to do so.
Info on traction created by skin wrinkles http://www.sciencemag.org/news/2013/01/wrinkles-help-fingers-get-grip