There are a number of issues at work here.
First of all, unless you have sampled a modern fuel injected (preferably DI), power-valved, reed valve induction, computerized two-stroke, you really have no idea how good a two-stroke can be. Deespite receiving comparatively little development money compared to four-strokes, the newest state-of-the-art two-strokes (Bombardier's E-TEC desing beign the best examples) are economical, powerful and clean with a very linear power delivery.
Also, in some ways, two-strokes are penalized due to their higher power potential, often being called upon to compete with four-strokes while giving up as much as a 50% displacement advantage (have a look at the displacement forumlas for motocross to get an idea of the overwhelming advantages held by four-strokes). Virtually all of the so-called peakiness of two-strokes comes from the state of tuning required to extract the impressive power figures from two-strokes. Imagine tuning down a Polaris 800 snowmoible engine from its present 150 hp level to the power level of an ATV engine, say 70. The resulting engine would have a powerband as wide as the Great Plains. Need further proof, have a look at an Evinrude E-TEC boat engine compared to a four-stroke competitor (the marine engines being much more mildly tuned, the E-TEC hand the four-strokes their respective asses in most cases).
Alternatively, getting similar specific outputs out of four-strokes (ie. @ 200 hp/L), curiously enough, results in four-strokes that are.... cammy. Look at a modern 600 cc superpsort motorcycle as an example. They have to rev them over 15K to get the kind of numbers 600cc two-strokes get at 8000 rpm. This further proves that, contrary to popular belief, four-strokes do NOT produce more torque (if they did, those little 600s wouldn't have to spin so high now would they?)
One place where two-strokes still fall short is, generally speaking, in terms of reliability and logevity; an engine with its piston getting heated every stroke is simply not able to last as long as, or run as reliably, as one whose pistons get heated every second rotation.
DI two-strokes are more fuel efficient, more powerful, lighter, cheaper to produce, easier to maintain than cammers. Sadly, until people's outdated perceptions change, the four-stroke will always hold the upper hand (notably on the sales floor, where it matters most), based largely on false or outdated facts. So, while two-strokes offer numerous advantages, particularly in anything approaching a high-performance application, no one will venture out to build them if they sit in showrooms collecting dust, and that is the ultimate reason.