Because open source is not really sustainable. People act too entitled. Everyone is basically looking for a big banquet of all you can eat buffet but someone else has to foot the bill. Of course Nim,D,Crystal cant do that. They do not have the mega corporation paying top dollar for all the engineers. There's no momentum.
But really even Google cant push Dart as far as they would and Rust is not such a success like people want you to believe. Yes there's more Rust positions out there than D positions but probably less than Go positions. Why is that ? Because Rust is hard and the costs of investing in Rust are steep. I would say the language has failed when there are no developers anymore interested in writing in that language and when there are no users using any services or products that used that language. In a sense there's very few languages that had failed. There were times when people doubted almost any language less popular than Java (or now JavaScript) as failed languages. Heck people even considered C# and PHP failed languages. I think people are just frustrated they dont get to use their favorite language at work. That's all.
