Because SPDY "looks like" SSL at layer 5, while SCTP does not: common deployments of Squid (one of the most popular HTTP proxies on earth) will pass through SPDY if they allow SSL, without any config changes.

On the other hand, SCTP has much stronger interactions with basically any deployed firewall/proxy anywhere, lacks widely used implementation for popular operating systems (Windows!), etc. For many networks, deployment of SCTP might mean replacing millions of $ networking hardware.

There are billions of dollars worth of ASICs in the wild designed to accelerate processing TCP.

There were billions of dollars worth of silicon in the wild designed to execute PDP-11 microcode.

For one obvious reason, SCTP won't make it through firewalls. Part of the design goal of SPDY is to minimize changes at every other layer of the web protocol stack.

SCTP doesn't pass through NATs.

Changing the transport requires patching the kernels of billions of installed devices. SCTP is not available on Windows, Mac, or any popular mobile device I know of.