Hey there, I've been googling for the past few hours, but still I couldn't figure out an answer.
My prediction: It's because water molecule, with the oxygen being a nucleophile, can potentially attack the sulfur entity of the starting material, resulting in side products. Chlorine is a good leaving group is my reasoning.
Starting material:
http://www.tciamerica.com/common/img-structure/A0074.gifRight?