In a strict sense they are, I think: A[1, 2:6] could writtenas an S-expression---something like (getindex A 1 (range 2 6)), maybe, but it is not in itself an S-expression. (I think: I'm relying on Wikipedia for the precise definition of "S-expression".)
Your broader point stands, though: I can't see any reason that one couldn't have S-expressions + syntactic sugar (not that I know anything at all about designing programming languages).
My guess is that it's a sociological thing? As scott_s pointed out, people expect something Matlab-like. From http://julialang.org/ : "Julia is a high-level, high-performance dynamic programming language for technical computing, with syntax that is familiar to users of other technical computing environments. "
Your broader point stands, though: I can't see any reason that one couldn't have S-expressions + syntactic sugar (not that I know anything at all about designing programming languages).
My guess is that it's a sociological thing? As scott_s pointed out, people expect something Matlab-like. From http://julialang.org/ : "Julia is a high-level, high-performance dynamic programming language for technical computing, with syntax that is familiar to users of other technical computing environments. "