"Why do I see pics of very nice Siberian elm by European bonsai artists ?"
I think it is because the species hasn't been used for as long in Europe for bonsai as it has in the U.S. The species takes time to inflict the emotional wounds that make people hesitant to use it
I never saw pictures of Europeans using the species until recently, like in the last five years or so.
In the U.S., where the species was planted over vast tracts after the dust bowl, it is much more available, familar and collected for bonsai. I have articles written on the species dating back 20 years or more in U.S. bonsai publications.
The species drops limbs without warning. That unfortunate trait isn't really noticed UNTIL a branch you've developed over a decade just withers and dies for no explanation. This trait has also made many of the most knowledgeable U.S. arborists call it THE worst tree to grow. It regularly drops huge limbs, sometimes on people and property, in the areas it has been planted in the U.S.
The species has all the positive elm attributes --nice bark ,finer twigging and an extremely hardy consitition (hardy to Zone 4) but with that single significant bad habit, it's an unstable species to work with. Doesn't make it unsuitable bonsai material, just a potential pain in the ass or a heartbreaker.