I'm not sure why it wasn't chosen, but I can give you some arguments against it. The braces introduce line noise. They introduce a new scope. I believe that rustfmt will currently put a newline after the opening brace. And it has the general problem that it can't be read from left to right.

(P.s. I'm not trying to start a debate, just trying to answer the question.)

I think it has to do with "ergonomics", and the existance and interaction with the "?" keywords played a big part of that.

Try writing a chain of async member accesses and calls with . and ? like that.