That used to be the case until Yamaha released the YZ400f back in 1998. That turned into a YZ426f in 2000, and then into a YZ450f in 2003. From that point on, the other manufacturers caught on and eventually released their own competitive four stroke bikes. Keep in mind that I'm mostly referring to motocross, but for the most part, the whole off-road community is now dominated by four strokes. As far as two strokes go, out of the "Big Five" (Yamaha, Honda, Suzuki, Kawasaki, and KTM), only two of them release two stroke bikes to the US, with those being Yamaha and KTM.
The old 125cc classes from yesteryear are now dominated by 250cc four strokes and the old 250cc classes are now dominated by 450cc four strokes. They gave the four strokes a very unfair (especially now that the technology keeps getting better) advantage regarding engine displacements. The old 125cc two stroke isn't even close to being competitive against the 250cc four strokes they have to run up against and one hasn't been ran professionally in a close to five years. Every once in a great while, there will be a local pro trying to qualify for a Pro National on a 250 two stroke in the 450 class, but it's been a few years since one of those guys have qualified.
