Because pure SCTP can't survive outside your LAN, thanks to everything in-between you and your destination. Why not use SCTP on top of UDP? Well, because one of the main benefits of QUIC is TLS being at its core.

SCTP you're talking about runs on top of DTLS on top of UDP. DTLS has issues on its own, but even if it didn't it wouldn't beat QUIC in TTFB.