Why is SCTP not much used/known

Indeed, SCTP is used mostly in the telecom area. Traditionally, telecom switches use SS7 (Signaling System No. 7) to interconnect different entities in the telecom network. For example - the telecom provider's subscriber data base(HLR), with a switch (MSC), the subscriber is connected too (MSC).

The telecom area is moving to higher speeds and more reachable environment. One of these changes is to replace SS7 protocol by some more elegant, fast and flexible IP-based protocol.

The telecom area is very conservative. The SS7 network has been used here for decades. It is very a reliable and closed network. This means a regular user has no access to it.

The IP network, in contrast, is open and not reliable, and telecoms will not convert to it if it won't handle at least the load that SS7 handles. This is why SCTP was developed. It tries:

• to mimic all advantages of the SS7 network accumulated over the decades.
• to create a connection-oriented protocol better than TCP in speed, security, and redundancy

The latest releases of Linux already have SCTP support.

SCTP is not very much known and not used/deployed a lot because:

• Widespread: Not widely integrated in TCP/IP stacks (in 2013: still missing natively in latest Mac OSX and Windows. 2020 update: still not in Windows nor Mac OS X)
• Libraries: Few high level bindings in easy to use languages (Disclaimer: i'm maintainer of pysctp, SCTP easy stack support for Python)
• NAT: Doesn't cross NAT very well/at all (less than 1% internet home & enterprise routers do NAT on SCTP).
• Popularity: No general public app use it
• Programming paradigm: it changed a bit: it's still a socket, but you can connect many hosts to many hosts (multihoming), datagram is ordered and reliable, erc...
• Complexity: SCTP stack is complex to implement (due to above)
• Competition: Multipath TCP is coming and should address multihoming needs / capabilities so people refrain from implementing SCTP if possible, waiting for MTCP
• Niche: Needs SCTP fills are very peculiar (ordered reliable datagrams, multistream) and not needed by much applications
• Security: SCTP evades security controls (some firewalls, most IDSes, all DLPs, does not appear on netstat except CentOS/Redhat/Fedora...)
• Audit-ability: Something like 3 companies in the world routinely do audits of SCTP security (Disclaimer: I work in one of them)
• Learning curve: Not much toolchain to play with SCTP (check the excellent withsctp that combines nicely with netcat or use socat, 2020 edit: nmap supports it for a few years now )
• Under the hood: Used mostly in telecom and everytime you send SMS, start surfing the net on your mobile or make phone calls, you're often triggering messages that flow over SCTP (SIGTRAN/SS7 with GSM/UMTS, Diameter with LTE/IMS/RCS, S1AP/X2AP with LTE), so you actually use it a lot but you never know about it ;-) 2020 edit: it's being removed from the core 5G network (no more Diameter, HTTP/2 instead) and will be only used in the 5G radio access network between antennas and core.

Reading the SCTP Wikipedia page I'd say that the main reason is that SCTP is a very young protocol (proposed in 2000) that is currently unsupported by the mainstream OSs (Windows, OS X, Linux).

If "very young" seems inappropriate to you, think about IPV6: "in December 2008, despite marking its 10th anniversary as a Standards Track protocol, IPv6 was only in its infancy in terms of general worldwide deployment."

It might not be well known, but it's not unused. Quite recently there was a draft published at the IETF about Using SCTP as a Transport Layer Protocol for HTTP.

Sctp is born too late, and for many situation TCP is enough.

Also, as I know most of its usage is on telecommunication area.