Two strokes are, literally, more bang for your buck. Combustion on each crank revolution means power, and three moving parts means very little internal friction. With the same development resources given to four strokes, there's no reason why two strokes couldn't meet all the current emissions regs, in fact several tuners, one of them notable Lotus, have developed multi-cylinder two stroke engines that meet regs, produce significantly more power and torque across the rev range than a four stroke of equivalent size, and are approaching four strokes in terms of fuel efficiency. Problem is, they have moved out of favour, sound odd (although I love the sound of both), and Honda have phased them out. The death knell has been sounded, so enjoy them while they still exist.
I used to be a four-stroke only rider until I got my KTM 200EXC. How such a tiny, light lump can make so much torque and bring so many grins still amazes me.