I have to agree with Ashi, previews get everyone worked up and more impatient for the release. Plus, when they do full reveal previews it leaves little excitement for the discoveries on the day it comes out. Everyone knows what's going to be there already. It's more fun opening a present when you don't know what's inside.
Ok yeah.. its more fun opening a present when you dont know whats inside. But getting a preview or not getting one might be the same. Why? Well.. devs usually update Steam earlier and they launch the pack after few days. So that means.. people can search the files and find out what are the new itms.. Because when steam or android/ios get updated the pack is not yet in the game... and everyone will search the files and find out the new itms they might even post them on forum. So basically getting a preview or not getting one is the same.