I think Dart really suffered from missteps in the early days. Originally, Dart didn't seem like anything special and it kinda wasn't. It was a dynamic language where optional type hints were suggestions and it'd be run in a VM just like JS and seemed like JS with some slight differences.

It feels like Dart became a completely different language that just happens to have the same name. It's now statically typed, has AOT compilation, null safety, etc.

I think that left Dart in a weird place in terms of mindshare. A lot of people likely looked at Dart 1, didn't see a real place for it, and haven't gone back to it. Google was also highly ambivalent about Dart for a while and most of the mindshare went to Go (despite the fact that I suspect most would prefer Dart).

In the end, it feels like Dart is lacking the ecosystem that other languages have. There's so much written for JS or Python or C# by comparison and so much of Dart seems to be in order to support Flutter.

Rust somewhat occupies its own place in the universe: people who want something a lot stricter than ordinary static languages where there's still a GC and still some runtime stuff. However, I think if Dart got more momentum it could be seen as a nice alternative to Java, JS/TS, C#, Python, etc. Note: I'm not saying that Dart has no momentum, but as you note it's mainly for Flutter.