Is this premise really accurate?
If it is, I think that the weight / power aspect may be attributed to the fact that the early dual sports were more off-road oriented, with a few owners being silly enough to ride their "enduros" on the road, versus today the dual sport market of motorcycle manufacturers gravitating more towards "Adventure riding", which seems to incorporate far longer highway jaunts to get to the dirt...
Anyway, my 99 KLR is 10 times better both on and off road than my 76 XT500 ever was, and my 07 Super Enduro will rip the lungs out of either of those machines, on or off road, and my 08 TE 610 will leave them both for dead up a rocky / sandy gnarly trail.
I like the way the dual sport market is going.
More choices, more specialization means that a person can find what they think is the ideal dual sport machine for their style of riding.
The potential WR250R buyer won't have the same riding goals as the potential F800GS buyer, and that is good for both the consumer and the manufacturer, I reckon.
Pure numbers alone (neither weight nor peak horsepower) don't mean a whole lot in the real world, I reckon.
PS Give me a great suspension over more horsepower every time.