Disclaimer: I work for Google and write dart for my current project.
Simple reason is there is a lot of code written in dart that is making money so there is continued support for it.
It's also a clear counterexample to the idea that Google always abandons projects.
its coming don't worry
Actually Dart's low adoption is an even bigger mystery in itself.
how is it a mystery? it seems quite clear the a language made to appeal to people who don't like taking risks in their programming languages would not want to learn yet another syntax for another language with the same semantics... What I find a mystery is how google doesn't get that