Evaluating conditions don't introduce bindings in COND. In Lisp typically bindings are only introduced in forms together with scope: DEFUN, LET, and a lot of other constructs. Binding construct which affect forms outside its syntactic scope is rare in Lisp.

In Scheme there is sometimes:

    (define (foo)
      (define a 10)
      (+ a 42))

But even there all the embedded "define"s, by standard, need to be at the top of the body, directly at the start.