Some are motivated by the usual "let's make our network faster/more secure"; however, SCTP is actually adding more capability.
Here's the original UDP protocol description: RFC 768. It's very short and to the point.
Here's an extension, DTLS, designed to retrofit security into UDP: Datagram Transport Level Security. It's considerably longer.
And then there's the successful RTP, a thin layer over UDP that adds sequencing: RTP: A Transport Protocol for Real-Time Applications.
Finally, here's a discussion from Google about doing SPDY-like activities (see next section for SPDY) over UDP: QUIC (SPDY over UDP).
But beyond TCP and UDP, what about just adding another type of IP, to meet other needs? Here's a discussion of SCTP, a sibling of UPD and TCP, that harkens back to some of the ideas of sequenced packet protocols. (Viz., RTP, ??)
Building over TCP
With respect to TCP, we are seeing new protocols that, instead of trying to change the fundamentals of TCP itself are instead trying to increase the performance of application level protocols, like SPDY: SPDY draft