The number of cams used is only one of many variables that determine how much power a given displacement engine can produce. DOHCs have the advantage of allowing multiple valves to be placed on each side of the chamber, enlarging the amount of flow that can be accomplished in & out of the cylinder, but depending on the configuration of the piston top and the cylinder head that may or may not be beneficiel when one is also considering gas mileage as a goal, especially with the proliferation of forced air devices. And with direct injection becoming common, one has to find room for the injectors in the cylinder heads as well as the spark plugs. Two valves might be able to provide for the engine’s need just as well as three, yet still leave room for the injector.
CAFE requirements and emissions requirements have taken over as the primary drivers for engine design. Yes, designers try to eek out power when they can, but mostly they try to move closer to 54 mpg and zero emsissions. Putting the injectors directly in the cylinders allows more complete combustion, even if it does reduce the space available for valve area.