I suspect one of the main reasons for using Lisp/Scheme style notation, is that almost all GPU programming anyone would want to do are (based on) mathematical equations.
For mathematicians, a Lisp notation is actually a lot more logical and easier than a C notation. (At least the older ones :)
The whole concept of iterations in calculations is a bit awkward in C (with all the parenthesis, yes...) in comparison to Lisp (where they're fairly well delineated blocks if properly indented)
Yes, you can mostly do the same in C, but I'd bet most mathematicians have more experience with ADA/Lisp than with C :)
Chicken/Egg sort of thing, then, now that I've typed all this out.