I know syntax isn't everything in a language but it's pretty important. There are much better (IMO) ways to express yourself than the approach Dart takes.
So it appeals to the widest (and thus) more average programmer.
Languages have an implicit audience and purpose. For example I consider Java a language designed to be consumed by code-monkeys. The mentality that you have a large enterprise project and you look to India to hire 200 programmers to finish it for you. The language has to have static typing, it has to be familiar to those learning previous languages (C++, C), IDEs are the preferred work environments, everything is very explicit and verbose.
Take Haskell, it is a language that would appeal to an academically minded developer. Someone who knows about catamorphisms, uses words like endofunctor, or say wouldn't mind struggling for weeks to comprehend what a Monad is.
Those are 2 different audiences. Neither language is better or worse, they are just tools and in some cases one is a better fit, in other case another one is.
Dart is closer to the Java.. It wants to be used in large code bases, by many (including average and junior) programmers. So adding things like fancy list comprehensions, de-structuring assignment, pattern matching and stuff like that, might not fit with its goals.