Probably so that it can be changed more easily (by redefining 'make-br-fn rather than hacking the reader).
;arc.arc from Anarki
(mac make-br-fn (body) `(fn (_) ,body))
I recall at one point people talking about making brackets work so that, for example, [+ _x _y _z] = (fn (_x _y _z) (+ _x _y _z)). It would extract all arguments from the body beginning with _, alphabetize them, and insert them into the argument list. I don't know if Anarki actually did this, but I do believe it accepted [+ _1 _2 _3 ...] at one point.
Nitpicking: that's not ssexpansion. Ssexpansion is short for symbol-syntax expansion, in which the occurrence of special characters in symbols tells the arc compiler to transform them, things like foo.bar -> (foo bar), foo!bar -> (foo 'bar). The brace transformation is actually accomplished via changing the reader table of the underlying scheme, and so is translated at read-time rather than compile-time.
This is important if, eg, you're writing scheme code to integrate into the arc runtime in some fashion; normally, in plt scheme, braces  and parentheses () are interchangeable; but arc changes the way  is parsed, so you basically have to avoid using them in scheme code. Whereas ssyntax is parsed by the arc compiler, not the reader, so you don't need to worry about it.