Mostly because WireGuard (intentionally) didn't bother with obfuscation https://www.wireguard.com/known-limitations/
> WireGuard does not focus on obfuscation. Obfuscation, rather, should happen at a layer above WireGuard, with WireGuard focused on providing solid crypto with a simple implementation. It is quite possible to plug in various forms of obfuscation, however.
This comment https://news.ycombinator.com/item?id=45562302 goes into a practical example of QUIC being that "layer above WireGuard" which gets plugged in. Once you have that, one may naturally wonder "why not also have an alternative tunnelling protocol with <the additional things built into QUIC originally listed> without the need to also layer Wireguard under it?".
Many design decisions are in direct opposition to Wireguard's design. E.g. Wireguard (intentionally) has no AES and no user selectable ciphers (both intentionally), QUIC does. Wireguard has no obfuscation built in, QUIC does (+ the happy fact when you obfuscate traffic by using it then it looks like standard web traffic). Wireguard doesn't support custom authentication schemes, QUIC does. Both are a reasonable tunneling protocol design, just with different goals.