I'm not sure if it applies to these particular tunnels as there are multiple methods to build tunnels using IPSec, but point-to-point IPSec tunnels often have lower byte overhead as you don't necessarily need to encapsulate in another type of packet, whereas the minimum Wireguard overhead is already significant. The biggest source of headaches with VPNs is the small margin between maximum and minimum effective IPv4 packet size, and when you cross these margins packets tend to get silently dropped.

Also, as this tunnel interface still integrates with OpenBSD isakmpd (IKEv1) and iked (IKEv2) daemons, you can use certificate-based authentication without installing some bespoke, proprietary agent for key management.