Hello,
As has been said The modern Moto Cross / Enduro Bike is a lot more complex than a 1970s bike , so that is most likely why the weight has not decreased over the years.
Also I may be wrong but was there a minimum weight rule for Moto Cross / Enduro introduced in the early 1970s ?
I think to prevent manufacturers building ever lighter {but fragile/dangerous} bikes.
Trials bikes seem to have lost some weight over the years, I have a 1976 OSSA MAR & a 2010 GAS GAS 300.
I dont know the proper weights for these 2 bikes , but just from trying to lift them the OSSA is noticeably heavier !