In your original post you said species
"that have been readily breed (bred?) in Europe but not the U.S."
I took that to mean species that are bred on a regular basis in Europe but not on a regular basis in the US. Did you mean like never in the US? If so, that's hardly a fair comparison.
Sure, some Gonyosoma have been produced in the US but hardly any. I have corresponded with many of the breeders who have accomplished it and most were only able to do it a few times.
I don't know anyone in the US who would suggest they breed readily. Well I know of one zoo, but ironically they kept them in huge walk in cages that were heavily planted and had rain systems that went off several times a day. Not mist systems, but simulated rainfall. That's more of a European approach. And they are bred in Europe and evidently can be found with not a whole lot of trouble.
Did you know that in Germany something like a Yellow Ratsnake is required by humanitarian laws to be kept in a cage that is 5'x5'x5'? If a pair is kept (or Gonyosoma) in the cage the cage should be increased in size by 10%.
But to the rear-fang issue, keep in mind that US breeders produce huge amounts of very dangerous cobras, rattlesnakes, etc. They are easy to keep and breed, which I can say from experience. So if it were a restriction issue certainly it would also appear in the more dangerous species, right? But I can buy a CB cobra for $45.
I also think there are examples among the venomous snakes that would reject the interest point I made earlier. There are many species of venomous that are very popular in the US but not bred very often. Yet many of these are supposedly bred in Europe.
But I won't say that for sure since I no longer correspond with European venomous keepers. At least not those who keep the real dangerous stuff.
Sorry, but I do firmly believe that European breeders are ahead of the US in breeding harder to keep species. How much of that is due to naturalistic vivaria is hard to say but I do suspect it plays a role.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
1.2 Gonyosoma oxycephala - Jave local (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)
1.2 Gonyosoma janseni - Celebes locale (Black & Tan)