How come fresh-layed carpet is such a conduit for static electricity, resulting in electric shocks?

It is not always fresh-layed carpet that generates static electricity. It can happen with new or old carpet at any time but generally always in dry conditions..

We usually only notice static electricity in the winter when the air is very dry.

During the summer, the air is more humid so the water in the air helps electrons move off you more quickly, so you can not build up a big charge.

When carpet is manufactured, it is protected from the elements in the factory.

Before it is laid it generally has been stored in a roll clad in a plastic or tar based paper to keep it clean and to keep water out so it is generally dry upon laying..

Thus the dry condition of the newly laid carpet is condusive to the creation of static electricity shocks.

When Laying the carpet the material is stetched by rubbing out the creases, most carpets contain fibres which when rubbed generate Static. Synthetic carpets can be highly charged after they've been manufactured and just unrolling from the main roll can produce shocks.