> And some Scheme code:

    (if (cond)
        (begin
          (preamble)
          (define a 5)
          (do-stuff a))
      (do-stuff 6))

This is not defining a local variable in Scheme. The various Scheme standards also require that DEFINE appears at the top of a body.

> Common Lisp wants you to declare every set of intermediate variables in a nested scope

Lisp programmers edit code by list operations. For example, I can set the cursor between (preamble) and the (let ...) form. control-meta-t transposes the lists. The let form is moved upwards and the enclosed body is moved, too. Try that in the C code. Code transformations are vastly easier with explicit scopes.

JavaScript has introduced a LET for a reason: block scope is clearer than VAR (function / global scope).

    (defun foo (a)
      (if a
        (var b a))
      (print b))   ; what's the value here?

In Common Lisp above would not be valid.

I would need to write:

   (defun foo (a)
     (let (b)
       (if a
          (setf b a))
       (print b)))

I can then immediately see that each B is inside a LET scope, which defines it. It's just by simply moving upwards in the expression tree. I would not need to search the whole expression tree. Also languages may do different things with definitions inside conditional expressions.