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Short story is; the radioactive ingredients (lanthanum, etc.) are added to the glass mixture itself, to give it certain beneficial properties (namely refractive index, I think).
More than likely dispersion relationship, which would manifest itself in the complex (i.e. frequency dependent) refractive index. Titania (TiO2) has a very high refractive index at some frequencies, but I don't remember it having as "nice" a dispersion relationship as other metal oxides. TiO2 also wants to agglomerate and crystallize, so it's probably harder to work with than older materials.
In human speak--the old glasses were likely easier to formulate and get a good color spectrum. This is mere speculation, and I'm not too keen on going out into my materials books to look up a bunch of ceramics to make a better postulation, sorry.