I think that languages like this don't go mainstream because people like/want to write code, and anything that does not help to actually write code is seen as an hindrance. The main feedback I've heard about this kind of tool is "it's clunky, it's a pain to write code with". Guess what ? It's a good thing IMO.
For this to go mainstream, the whole software industry would have to go through a major revolution by shifting its focus from "writing code" to "deliver a working product". We should acknowledge that "writing code" is one tool among others, and may be not even the primary one (I personally think that specification engineering is the most important one).
In the field of critical embedded software, this has happened that's why it took off.
Another clue of that is for instance the Event-B/Atelier B that is intensively used in the railway/metro world (Paris line 14 was made with it, 2 bugs found in 15 years). While doing B, you get a broad specification you refine along the way into smaller problems. Writing code is actually the last step you do, when you are sure that all pieces fit together.