This is complete noob conjecture:
I think it has to do with it being so thin. If you can lay on a relatively thick coating during the enameling / heating process, it will be far sturdier and easier. But with something 1-2 mm thick, any flaw or unevenness is going to result in a junked dial. Scottish microbrand anOrdain - who specialize in enamel dials - has said something like half of their dials are junked between flaws that show up during the baking process or during the application or printing of indices / logos / etc.
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