A-expressions are a choice, and it is fine to argue whether it is a good one. There are plenty of other ways of achieving honiconic syntax.
I have heard of M-expressions but never of an actual implementation. Scheme also seems to have some SRFIs involving alternate syntax that's whitespace dependent, like SRFI 119 (wisp), SRFI 110 (sweet-expressions or T-expressions), or SRFI 49.