Without QUIC, the Ethereum network would have to rely on libp2p's TCP based transport. This transport involves a lot of moving parts (and network roundtrips) to set up a connection, namely:
1. multistream to negotiate a security protocol
2. security handshake (in Ethereum's case: Noise)
3. multistream to negotiate a stream multiplexer (mplex and yamux)

In addition to wasting a lot of network roundtrips, all of these components are less well specified and less well tested than QUIC.

Furthermore, widespread QUIC support makes it easier for new implementations to join the network, as all they need is a QUIC stack (which are available in every programming language by now), as opposed to the bespoke libp2p handshake described above.