Why is an RSA public key BITWRAP-ed in ASN.1 DER encoding?
That is a Subject Public Key Info structure defined by the X.509 standard:
   SubjectPublicKeyInfo  ::=  SEQUENCE  {
        algorithm            AlgorithmIdentifier,
        subjectPublicKey     BIT STRING  }

The reason it's BITWRAP-ed is that the structure of the wrapped public key depends on the algorithm. It could be a RSA public key as you described, but it could be an elliptic curve or DSA public key too. Without wrapping, the ASN.1 parser would need to be aware of the algorithm it has just parsed to know how to parse the subjectPublicKey, which is cumbersome. With wrapping, it just parses the whole structure and subjectPublicKey can be interpreted later.
(The big question for me it's why it uses a BIT STRING instead of a OCTET STRING...)