SCTP still have some donwsides it has to resolve https://http3-explained.haxx.se/en/why-quic/why-tcpudp#why-n...

Plus we need happy eyeballs for transport if SCTP run over IP and not encapuslated https://datatracker.ietf.org/doc/html/draft-grinnemo-taps-he

But IPv4 pretty much non-workable since most end-users behind NAT and there no known implementation to work around that.