The double displacement rule was always in effect as far as i remember.At least in the original 250 class.If memory serves me the displacemnt limit used to be 500 or 550 for the old style 4 strokes.Once Yamaha came out with the 400f and the rest of the manufacturers started making the new style f1 based 450's and told the buying public they are the very best that is when the 4t's took over.The buying public falling for the latest and greatest and not buying 2t's is what killed the 2t.At least here in the USA.
The double displacement rule came into effect in the early to mid 1990's as I understand... I have looked for the rules and have not been able to find and confirm this though. If someone does know, please post it here.
At the time the rule was passed no one thought that the 4 strokes would beat the two-strokes even with the displacement advantage. Then a lone Yamaha engineer took it upon himself to build the very first modern four-stroke motocross bike. When he brought the idea to his bosses they just laughed at him at first... t took a while before they got behind the machine...
Then when the four-strokes were still having problems beating the two-strokes, the AMA put into effect the unleaded fuel rule, which gave the four-strokes even more of an advantage.
The first racers to switch over to the four-strokes were the vets... it was easy to ride. Then as more and more showed up at the track and began doing well, more people bought them and bought into the whole four-strokes are better marketing hype... sales of two=strokes all but disappeared.
Over the past two years there has been a shift in this situation... while not every single four-stroke is a time bomb, enough were to force some people out of the sport all together and others to seek out alternatives. The two-stroke has begun to look like a great machine again. Light, fast and fun...
2010 will be a banner year for two-strokes... it's happening already!