In IKEv1 (RFC 2409), preshared secret is involved to key derivation where IKEv2 (RFC 7296) use it for only authentication. When we consider post-quantum security, this property makes IKEv1 suitable if preshared key has sufficient entropy. Therefore post-quantum extension of IKEv2 proposed in (RFC 8784) which is basicly adding an additional secret that is shared between the initiator and the responder in addition to the authentication method that is already provided within IKEv2.
Why pre-shared key is not involved to key derivation in IKEv2? Are there any disadvantage of using it as in IKEv1?