iTunes Pre-orders

I have been wondering for a while now why for the majority of pre-order albums on iTunes now you cannot listen to the previews of all the songs. They seem to just allow the single and maybe one or two others to be previewed.

Does anyone know why this is? I am sure they pretty much all used to be available to listen to all of the tracks preview for either 30 seconds or 1 min 30 seconds.

I guess maybe they don't want to give too much away.