Various theories hint at the interests of the oil lobby to continue four-stroke dominance (just look at the low mpg of most american manufacturers in general) and perceived customer comfort being the most widely used trump. High fuel efficiency does not usually provide sporty acceleration, low engine noise, and high torque at low revs.
That being said, no doubt many consumers don't care as much about that as the marketing departments of the automotive industry. In reality, noisy diesels have sold well in Europe (thanks in part to diesel fuel subsidies) and customers have bought poor performing, smaller cars for everyday use. They just don't make big margins on cars that sell for less than €8000 new. So once again striking a balance between shareholder interest (increasing profits) and global economic / ecological interest (decreasing emissions and oil reliance both by better fuel efficiency and better combustion of cleaner, more varied fuel) is an impossible mission.
Until oil prices go up, don't expect any good technology to prevail. The four stroke petrol engine will die, but not before oil costs increase.