Good question. This experiment is all about controlling the flow of electrons. Wood is an insulator. Meaning it will not easily conduct an electrical charge. Some other common insulators are glass, rubber, air and plastic. However, wood is the best at not building up a static charge on the surface like glass or plastic and you need one other requirement, a sturdy platform to work on. The wood table provides all 3 requirements: a stable platform to rub the acrylic sheet, it will not conduct electrons away from your acrylic sheet as you are attempting to build up a static charge and it will not build up a static charge on its surface as well as you are rubbing the acrylic sheet with wool.
For the results of your Leyden jar measurements to mean anything, you need to take care not to inadvertently discharge the electrophorus or have a source that could randomly introduce additional charge to your system. Wood is an ideal material to work on that will electrically insulate your experiment.
For a discussion on conductors and insulators visit: http://www.ndt-ed.org/EducationResource ... lators.htm
Hope this answers your question,
I hope this helps.
"As the circle of light increases, so does the circumference of darkness around it."
~ Albert Einstein