Not that it's an amazing reason, but H3 doesn't run over TCP, and running a UDP proxy doesn't sound like a great time.
QUIC, in particular, is harder to proxy (if you're load balancing, say: https://quicwg.org/ops-drafts/draft-ietf-quic-manageability....).
If it is point-to-point and you control both those points (forward A to B with ports open as approp), proxying any protocol should be straightforward, no?