Comment by Twisol

UDP pretty much just tacks a source/destination port pair onto every IP datagram, so its primary function is to allow multiple independent UDP peers to coexist on the same IP host. (That is, UDP just multiplexes an IP link.) UDP as a protocol doesn't add any additional network guarantees or services on top of IP.

QUIC is still "their own protocol", just implemented as another protocol nested inside a UDP envelope, the same way that HTTP is another protocol typically nested inside a TCP connection. It makes some sense that they'd piggyback on UDP, since (1) it doesn't require an additional IP protocol header code to be assigned by IANA, (2) QUIC definitely wants to coexist with other services on any given node, and (3) it allows whatever middleware analyses that exist for UDP to apply naturally to QUIC applications.

(Regarding (3) specifically, I imagine NAT in particular requires cooperation from residential gateways, including awareness of both the IP and the TCP/UDP port. Allowing a well-known outer UDP header to surface port information, instead of re-implementing ports somewhere in the QUIC header, means all existing NAT implementations should work unchanged for QUIC.)