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.
I implemented that feature (multiple automatic parameters, with alphabetic ordering, so [cons _y _x] = (fn (_x _y) (cons _y _x))) in anarki. I don't know for sure, but I would assume it's still there.
