Somehow Typescript convinced people from the Javascript side of the fence that they could mix both and eventually upgrade from it. While Dart also tried the same feat, it failed doing so.
Nowadays Dart is only being considered to anything because the team behind it are top-notch and implemented themselves a platform where Dart could be the king.
Giving their talent they were able to create a great platform, where they would made even more success if they used Javascript or Typescript as a development language. But giving they wanted to save all their years of work on Dart, and giving its not a bad technology per-se, it just had a adoption problem, Dart lives on Flutter.
If you want to use Flutter go for Dart, but picking Dart to anything outside Flutter will just alienate the developer crowd as giving even Typescript is some sort of a niche language in terms of adoption, nevermind forcing people to learn Dart.