I'm going to guess because nobody wants to deal with carbs anymore, and very likely wouldn't know how, its an 85 design and tech has moved forward by heaps and bounds since the first iteration of the Vmax. Some of the newer buyers probably missed out on the bit of history of the Vmax and dont even know what that is. Those of us that were doing this in the 80s probably understand it's iconic value. We remember that it was the fastest production bike for quite a number of years and how no matter what you rode, you didnt test it out against a Vmax. Not only that but they were sort of a rare sight even back then, at least here.
Also possibly potential buyers ride one and decide that it just fails too live up to the hype, when mine was out of tune it was considerably less impressive than it is right now, so if someone rides the wrong max at the wrong time they could just decide it's only fast by yesterdays standards or when compared to a 45 deg twin. I think quite simply it's a niche bike with a narrow pool of potential buyers. That and the fact that it's no longer gets press as the fastest thing on two wheels. It's one of those things that just needs the right buyer. It's definitely an undervalued gem. But it continues to be a heavy hitter in the power cruiser segment and will probably never loose it's iconic stature among seasoned riders.