Why so long?
I worked on THz transmitters 15 years ago. I'm amazed they're not in widespread use yet. These things were also supposed to revolutionise medical imaging (hand held "X-Ray" machines) as well. Now the only current use is allowing airport perv^H^H^H security to see through clothes.
I can only conclude that there's not much demand for GBit wireless or hand held body scanners, but there is a high demand for looking at people naked.