No good reason I know of... The "is" roughly maps to comparators in other Lisps that are designed for "near constant time" comparisons, which is possible for symbols and numbers.
Strings may have gotten grandfathered into this behavior because they were thought of as similar to symbols.
Another possibility is that a Lisp string implementations could, in theory, use pre-computed hashes for comparisons, which WOULD allow near-constant comparisons, while slowing other operations.
