It’s multi stream, reliable connections. WireGuard’s encryption over UDP is none of those things. WireGuard encryption is simpler and far more flexible, but also less capable.

I’m not advocating WireGuard’s transport be replaced with QUIC (they’re solutions for very different problems), but that doesn’t mean QUIC is saddled with legacy junk. Most applications want protocols that are connection-based and optionally offer retransmit - that’s not legacy junk, that’s just what is called for in most cases. L3 encryption is an unusual application in that it doesn’t call for these properties.