Probably because that's what most riders have come from and in the Uk they are by far the biggest biking sector.
As giles said so much is down to the rider, on track I used to lap at very similar pace on all the bikes I rode on-track (FZR400RR, TL1000S, TRX850, Laverda 750, ZX9R, Thundercat and a CB500)
All modern sportsbikes far exceed the riders talent, especially on the road, just look at the IOM a full blown superbike laps no more than 1% faster than a superstock bike - and that is with world class riders on-board.
On the road most of us would not corner much quicker on a modern sportsbike than we do on a GS, 99.99% of riders would definately be slower on a S1000RR round a track than a top racer would be on a GS.
With my aversion to 3 figure speeds (or at least the potential loss of job / jail time) I know I can travel from A-B on British roads as quick on the GS as I could on any other bike, and in relative comfort.
It has to be said that having a 180+ mph bike in the garage feels good and the full bore acceleration of a 1000cc+ bike is bloody funny (but only until you get caught)
In theory there should be no comparison, just like the Factory Superbikes should piss all over production bikes at the IOM.