[I'm on the Wing team]
The elevator pitch is that there are things we cannot do with existing languages, such as our inflights and their connection to preflights. This is because that if we used an existing language we'd have to support its legacy code and could not impose new limitations that are needed to execute the inflight system well. It doesn't mean that you cannot come close with existing languages, and there are nice projects that do that, it just means that you cannot go all the way.
The full version is here: https://www.winglang.io/docs/faq/why-a-language