Efficiency, emissions, weight, and packaging. More cylinders means more parts, such as more valvetrain components, more stuff rotating, etc. More cylinders adds more internal engine drag, which lowers efficiency. V6s are awesome in engine packaging (assuming there’s no turbocharger involved), as they are practically a perfect cube in the engine compartment. Their shorter size makes it easier to shove a larger transmission and maybe a hybrid motor or two. High revs are also not great in the emissions department.

And most importantly, most customers are the types who wants an instant kick in torque. Reving out the engine is seen as “lag” or “sluggishness”.