As a side benefit, this phosphor is moisture resistant, and UV light resistant. Promethium was discovered in the 1940's, and was available in gram size quantities by the early 1960's. Promethium-147 was first used to illuminate LCD watch displays in the 1970's. The X-rays are shielded by using leaded glass or leaded acrylic resin as a crystal. Also, sapphire crystals cannot be leaded.
Manufacturers do it because it is so cheap.
The last U. S. high neutron flux reactor at Savannah River was shut down in 1988. Medical isotopes that must be produced by neutron activation are made in Canada at Chalk River or in Europe. It will not be until the first half of the next decade before there will be a domestic source of high flux neutrons. This is now defined as 3.7 x 10 to 10th power disintegrations per second. The patent was applied for in 1978 and granted in 1981.