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.
Uh... explain then why European & Japanese manufacturers can make high mpg with the same four-stroke engine technology? Oil lobby aside, the technology has more efficiency possible.
And irrespective of that, two-stroke doesn't necessarily mean less fuel consumption - and is far more likely to mean higher lubrication oil consumption.
In reality, noisy diesels have sold well in Europe (thanks in part to diesel fuel subsidies)
Where on earth did you get the idea that Europe subsidises diesel?
and customers have bought poor performing, smaller cars for everyday use.
That's more likely to be a pattern of behaviour - distances between cities and key locations are smaller due to higher density, and roads are narrower in Europe, so having a massive car is more likely to be an inconvenience.