Paul, its possible that the variegation turned out to be unstable. That would mean the plants revert to plain green over time. A lot of variegated plants do this. The all-green leaves make more food from the sun so the plant produces more of those leaves until all the variegated ones are gone.
If you don't keep propagating the variegated form it disappears entirely. Nurseries producing those plants would give up fairly easily because its too work intensive to keep them going, and then not dependable.