This is because pubic hair will not be perfectly cast and will have an adverse effect on the final sculpture. There is also a risk that long hair get trapped in the molding materials making the removal of the mold very painful. A short fuzz will glide out painlessly.
Photo: Erik Frausin
You can obviously shave yourself but many salons and spas offer the service at reasonable cost. This coquetry has in recent time become very popular.
From my personal experience I can report that the waxing procedure itself is not very pleasant and can even be painful but I do love the smooth feel and sexy look of my bare pussy.