Likely has something to do with intended purpose. I'm going to speculate a bit. If anybody knows a source for such statistics, we might be able to verify.
I'm guessing that the intended purpose of most hand-gun purchases (excluding military and LEO) are, sorted from largest to smallest: (1) self defense; (2) target / plinking; (3) hunting. For self-defense, a firearm will be optimized for capacity and rapid fire, giving rise to a prediminance of semi-autos, with revolvers right behind. Other actions, such as the various single-shots, would be better used for target shooting, plinking, and hunting.
In contrast, I'm guessing that (again excluding military and LEO) most rifles are sold for (1) hunting; (2) target / plinking; (3) defense. For hunting, or target shooting, capacity and rate of fire will be far less important. Generally speaking, a bolt action will be more accurate than a semi-auto, especially for the same price. So you see the many actions that are equally well suited for hunting and target shooting.
Just thinking "out loud." But I'll bet one could correlate type of action with intended use and come up with a pretty well evidenced argument such as the one above.