Leaded glass was a cost-cutting measure, used only for the back and sides of CRT. Leaded glass gradually turns brown under X-ray exposure, so the front screen used barium and strontium instead. There's no reason the whole CRT couldn't use this kind of glass. There are probably other heavy elements that would work too.

(Early black-and-white CRTs running at lower voltage did use leaded glass for the front too, but they aren't the CRTs people interested in old games care about.)