Its simple really, performance sells bikes. The works Yamaha teams promote sales for fast bikes, the motocross promote sales of dirt bikes.
The sensible fuel efficient version is promoted by, er, er, nope. The small capasity 250ish commuter stuff sells its self to discerning punters who want 70-80 MPG.
The rest of the time as far as Yamaha are concerned with increacing MPG, is more Motorcycles Per Garage.
Its the punters fault, the mainstay of motorcycles are glorified toys. You must know somebody who can throw money at his/her bike for the latest fashion. Those people couldn't give a monkeys about fuel, they want BHP. The extra 3mph over the green one they had last year is all they care about.
I agree fuel economy on bikes is shite, my TDR can stretch to maybe 60mpg ridden efficiently, and that doesent mean slow. But that bike dates back to when bikes were all round items. Now they have been designed to fit a certain group. The common point in that group is the same thing we see on here week in week out. Want to go faster.
If our motorcycle engines were tuned to the same level as cars they would be better on fuel. To use the focus angle here:
That 1600 motor cranks out 113hp, or 70bhp/litre
My TDR250 gives 50hp, thats 200bhp/litre (the 50 is due to the pv changing the engine dynamics when its open your getting 30mpg or less when its shut you have maybe 20hp so 80bhp/litre and so economy)
Even the 90bhp divvy is making 100bhp/litre
No mater how these engines are made and by whom bhp increaces come from burning fuel, If any of the big four churned out a spanking new bike at say 600cc giving maybe 35hp that it was designed to for economy not gutted to 33bhp by a 'kit' would give good gas milage. Cant see there being many takers though.