because they are still not efficient under most of their rpm range and get worse away from torque peak, and they still burn oil.
2 strokes, while they have good power to weight, have poor power to fuel (bsfc), compare carb to carb, compression to compression. And the extra fuel per horsepower equates to extra pollution.
from
Using BSFC #'s
Code:
Engine @ 1,000 @ Peak @ Peak
Type RPM Torque Hp
4-stroke (low compression, carburetor) 0.62 0.47 0.52
4-stroke (high compression, carburetor) 0.60 0.42 0.47
4-stroke (high compression, closed loop) 0.52 0.35 0.42
4-stroke (supercharged, carburetor) 0.75 0.50 0.55
4-stroke (turbocharged, closed loop) 0.57 0.45 0.50
2-stroke (low compression, carburetor) 0.85 0.55 0.60
2-stroke (high compression, carburetor) 0.80 0.50 0.55
Diesel 4-stroke 0.35 0.25 0.30
Diesel 2-stroke 0.40 0.29 0.34