People watch car racing, downhill skiing and other such for several reasons. Sure there's national pride, appreciation of technology, awe at the human ability, but the apparent and visible speed is the biggest thrill of all.
Yachts can be exciting, the Volvo race certainly is, and the way it is presented on TV it can hold the viewer's interest despite the fact that the yachts are usually miles from each other. You get to see them battling through monster waves, meet the guys, see strange places, hear the inside story (maybe) and see the technology make it or break.
Problem with the AC is most of that is not visible, unless you are physically present to appreciate the scale of these things. Seeing a couple of incredibly large boats plow majestically through almost calm seas, on and on, just doesn't convey much of an impression on a TV screen. The size, technology, human effort is all there but it doesn't come through on the screen. They're just so big that everything else is overwhelmed by their scale, the waves and the people are so small as to be almost invisible, the speed is not apparent and the ending is inevitable after the first few seconds.
It's about as exciting as seeing the pyramids on TV. How big are they? Well imagine that! Lord, it made me yawn just writing about it. Now power boat racing IS spectacular!