Method for high clarity plique à jour
This is completely a guess on my part...but I know that when I was taking a jewelry design class in the early 1990s, my instructor had to eliminate the enameling section of the class because the "ingredients" and techniques had just been determined too toxic to teach, and she hadn't worked with the new resins enough to be comfortable teaching with them.  

So, maybe the big difference is our current scientific knowledge about the levels of toxicity that limits our access to the same raw materials that Lalique was using.