We could have, and discussed it, but IIRC there's some good reasons why we didn't. That was a very long time ago and I don't quite remember them now, though... I think that the idea is that this gets ambiguous with type ascription?
I'm not sure of the historical context, but it would have required either for the syntax to declare the return type of closures to be different from functions or for type ascription to have a different syntax.
To be honest, type ascription being : was a (small) mistake that affects the comprehensibility of errors because it is distance 1 from multiple common typos and is valid almost everywhere in the grammar, so it is very easy for the parser to go down the wrong route. Even worse, the feature is nightly only so we're not even getting the benefits of the simple syntax.
Advice for language designers: make your grammar redundant, with lots of sign posts for the compiler to recover when things are wrong. Things like mandatory semicolons are good for both humans when reading code and for the compiler when parsing it. You can also make your compiler perform semicolon autoinsertion, but still have an error, that way the type checker can still run.
