Because they tend to do very little of everything else.
CC? Darius and Volibear both have a single displacement based effect and a slow built in to their kits. Compared to any CC tank or utility mage, the amount of non-damage based commitment they can make to a team fight is exceedingly poor.
And then, how do they deliver their damage. You have the choice of semi-awkward AOE capabilities or somewhat more easy to apply insanely weak AOE capabilities. Take your pick, any mage will blast them out of the water in terms of big multi-striking damage.
And then there's that you often have to build into their defenses. Darius has no true defensive steroids and Volibear requires a large margin of resistances to make sure he doesn't die trying to utilize his. This means less flexible build paths which means they must specialize even more into the one job they're good at which means it's exceedingly hard trying to overcome what they lack through items. For reference, Utility Alistar does the tank job just as well as either of these guys with his ult up while also being able to afford item slots that more directly support the team.
And hell, they're both melee. Given how much the boards complains about it, it should not be that hard to realize why that's a problem.
The only thing these guys are good at are stat checking individuals in a fight. The reason why they are so prominent in combat stats is because their only strong points are their combat stats.