Why is the lead crystal glass transparent?

First off lead crystal glass is not made of 100% PbO.

Lead glass is potassium silicate glass which has been impregnated with lead oxide (from 12% to 28% by weight) in its fabrication.

The introduction of lead oxide in varing amounts was employed for several purposes the first primary purpose was to design a type of glass that could block x-Rays second it was found that lead also increased the refractive index of the glass, and thus improved lustre, or sparkle.

Lead glass’s primary contemporary use is in the construction of televisions and monitors. In a standard cathode ray tube, high-energy electrons are fired at phosphors painted on the inside of the tube. This impact creates the visible light the monitor or tv screen produces; it also produces X-rays. The lead in the glass acts as a barrier to this unwanted radiation.

When lead glass was artistically cut and used to make table ware such as wine glasses and decanters it was called lead crystal. Within the last 10 years it was found that alcoholic beverages stored in a lead crystal decanter was capable of leaching the lead from the glass and contaminating the contents.