From my perspective, it comes from being used to parentheses in English. Specifically, you have a man thought, then you have some other modifiers that may expand on that concept which you enclose in parentheses. This fits well with C style functions, for example. But with Lisp, the main thought (the function name) is also enclosed in parentheses along with all its modifiers, which makes it a bit harder to follow the code.

However, I do like the overall elegance that this brings to the code, where code and data are the same. Which is why I also like Postscript -- it does the same thing but in an RPN style (a function is actually an executable array).