In short, they are looking to be the go-to language for scientific and high performance computing. Programmers in that domain will have some expectations, and S-expressions are not it. They also consciously mimicked Matlab's syntax.
Julia does seem to use some sort of Scheme in their implementation, wouldn't it make sense to allow S-expressions in the language (or is it allowed ?) ? Or maybe, the designers/implementors felt that it wasn't necessary ?
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. "
Yes, they are. Nobody (ie. small amounts of somebody) likes Lispy syntax, except Lisp programmers (and those are not that many nowadays anyway).
Julia wanted a chance to be a succesful scientific language.