I'm sure those two things are correlated, but why would stock trading volume be a point of comparison? I have no idea, a priori, what that relationship would be in an ideal situation.

The point of prediction markets is that they aggregate the available information. If the price moves to roughly what people think is the right level, then there won't be much further movement, even if interested potential speculators are looking at it. You'd expect lots of movement on something where there's a lot of disagreement about the likelihood.