I know that one big deal is fewer round trips. DTLS+SCTP requires several round trips to establish a secure connection (around 4 I think). DTLS 1.3 should improve that on the DTLS side of things, but QUIC is designed from the beginning to have 0 round trips for many cases, and 1 round trip in the worst case. For high RTT scenarios, that can make a big difference.