The TCP congestion control heuristics are likely dated. Using an initial cwnd of 4KB is just too low for most users, and the data to increase this limit is compelling (see Chu/Dukkipati data). HTTP clients/servers are using multiple connections to achieve higher initial cwnds on the initial burst.
SCTP does centralize cwnd across all open streams, but it uses the same core algorithms as TCP and fails to address the initial burst. Unless the initial burst is increased, SCTP is slower than HTTP over lots of TCP connections out of the gate. This is measurable and demonstrable.
Oh, and SCTP doesn't work over NAT. So still waiting for IPv6.