It ended up in application layer because of http gateways and NATs. The reason why spdy was https only at google was because https is pretty much the only port that nearly all corporate gateways let through as a "black box"

Google even did tests with http over SCTP[1] and found that it solved a lot of the problems that spdy did. It's well accepted that the transport-layer is the "right" layer to fix this in, but it would not allow as wide a deployment.

1: http://en.wikipedia.org/wiki/Stream_Control_Transmission_Pro...