Because the boards are made from wood they are extremely heavy. A small wood board may weigh up to 20 pounds which means that you sink when you hit the water. Wood boards tend to be more popular on the east coast where the beaches are flatter and much of the skimboarding that people do takes place only on the sand. This isn't to say that you cant ride waves on wood boards or on the east coast. It's just harder and takes more practice.
Foam boards are very popular on the west coast and in Hawaii where skimboarders ride waves more.
In fact I have been skimming for 10 years and have only personally seen a handful of wood boards. Foam boards are light and buoyant. My board weighs just over 3 pounds. This is because the materials are more expensive and they are harder to make. All things considered, if you plan on trying to ride waves, you need to consider investing in a foam skimboard.