SCTP is relatively new. The first RFCs were published in 2000, but it wasn't until a couple of years ago that they were shipping operating systems with complete implementations of the protocol. For Windows and OS X, you still need a third-party implementation, but on most other systems it's included in the kernel.
A lot of existing application layer protocols would benefit significantly from being run over SCTP, and it's worth considering as a transport layer protocol if you're designing a new system today.
The Internet is still heavily based on protocols that were thought up back when 300 baud connections were state of the art and people could memorize the names of every computer on the network. Things like SCTP and IPv6 are gradually changing this, with incremental improvements that make various aspects better.
It's a testament to the designers of ARPANET how well their designs have stood the test of time, but it's getting to be time to move on. We can't tell everyone on the Internet to switch to a completely new system, but we can start using the improved protocols that are available and let the older ones die off as they fall into disuse.