Comment by tsimionescu

Because it doesn't work very well in practice. The parentheses are very small glyphs, so the colors are actually barely visible on them, unless you have an extremely thick font. And if you have an actually complex expression, it easily turns into color soup. If you really need a complex parenthesised expression, it's much better to use newlines and other means of spacially aligning the code, Lisp style, then having it all in one line with rainbow parens.