It's because the language was made to compile to JS, and competition to JS, so appropriate tradeoffs were made.
For example:
1. JS is single-threaded, with extremely heavy threads only recently made available. So Dart is single threaded, with extremely heavy threads available.
2. JS is weak-typed, so Dart was made optional typed. Remember, it was made before typescript, so they probably didn't expect that such type-heavy features as ADT would interest people.
Kotlin does the same (for example, internal immutability would be much nicer, but since the JVM doesn't support that, neither does Kotlin).
For example:
1. JS is single-threaded, with extremely heavy threads only recently made available. So Dart is single threaded, with extremely heavy threads available.
2. JS is weak-typed, so Dart was made optional typed. Remember, it was made before typescript, so they probably didn't expect that such type-heavy features as ADT would interest people.
Kotlin does the same (for example, internal immutability would be much nicer, but since the JVM doesn't support that, neither does Kotlin).