[int_19h]

Contrary to many of the responses you've got, the real answer is that it's just a convention with no particular reason other than historical cultural preference. For evidence, I present R: it looks like C syntactically, but all constructs actually desugar into function calls (even assignments!). And function calls themselves are also C-style - f(x, y) - but once it's parsed, the result is a "pairlist", which is exactly what it sounds like to any Lisper. So any R program is represented as nested lists, with constants and symbols as leaf nodes, exactly as in Lisp.