Lead glass is not only easier to cut, it is also easier to work when it is a hot metal.
However, it is more expensive than other sorts.
Casing
over silver foil would be very tricky, the hot metal would need to be quite cool to prevent the silver burning and reacting with it.
That gives rise to opaque blues and yellows appearing.
You can see that happening in earlier pieces of IoWSG Azurene.
You don't see that happening in these (earlier) Murano pieces.