Dart was relegated to effectively being Flutter. It was originally intended to supersede JavaScript but no other engine would commit to that. It failed at its initial goal and hasn’t really found a footing outside of Flutter.

Carbon is by its own admittance not ready to use and I think mostly relegated to solving Google’s problems with C++ right now.

Both of them didn’t ship with a standard library as robust as Go’s.

One thing that made Go popular out of the gate is it is extremely good fast to build out robust HTTP services and infrastructure.

This is a very common use case and they tailored Go to be a great fit for it. You can build your entire backend without a single third party module if desired using Go’s standard library and it isn’t terribly complicated to do so.