Not sure what gave you that idea. You're probably refering to standard def version of SDI. HD-SDI supports 720p/1080i through a single cable, up to 100 meters. Dual link HD-SDI can support 1080p.
People keep bringing it up because it has some technical benefits over HDMI, such as field termination of standard coax cables with BNC connectors, and the distances it can go. There is also no technical reason why HDCP over HD-SDI cannot exist. But there is a different reason why it did not gain traction in the consumer market.

HD-SDI is a non royalty bearing standard offered by the Society of Motion Picture and Television Engineers. SMPTE doesn't really benefit either way from a manufacture adopting it or not. It was never in SMPTE's agenda to take over the consumer digital video market.

On the other hand, Silicon Image had recognized the market opportunity, and leveraged their existing work with DVI for the PC world, and combined that with HDCP to offer their own standard. That combined with strategic partnerships with top tier CE companies like Matsushita, Philips, Sony, etc. has allowed them to take over the world of consumer digital video.

Is it the best technical solution? Perhaps not. However it takes more than the best technical solution to win the market.