From the official Flutter FAQ:

For example, when we adopted Dart, the language did not have an ahead-of-time toolchain for producing native binaries, which is instrumental in achieving predictable, high performance, but now the language does because the ***Dart team built it for Flutter.***

(Emphasis mine). I always felt like this was why Flutter chose Dart. If you could pick any language, why not choose one that will invest incredible amounts of developer resources to fulfill whatever your needs are?