Unfortunately, the same thing happened to trailbikes that happened to cars - over the years, they got heavier. Then motors got bigger to carry that extra weight, which made everything heavier still. Unlike cars, this weight had more impact on handling, especially on rough ground. It's not easy getting a 300 lb motorcycle over a log.
I'm still waiting for a modern version of the 1978 VW Rabbit which weighed 1880 pounds dry. With modern technology it should be able to get 100 MPG. In the mean-time, it looks like Yamaha may be thinking the same thing. It's only a concept, but it's headed in the right direction:
Yamaha shows retro lightweight 125cc motorcycle that gets 220 mpg
Bring it on!