That's actually a very good question.
The answer is similar to why non-speed demons ride liter-bikes: tractable power at low revs. You get to be lazier at shifting and not have to always be working to stay in the powerband.
My 1250GS has about 35hp more than the Norden. The boxer engine has incredible torque off idle. Hill climbs don't often require a downshift, whereas on the Norden, because of its peaky powerband, sometimes (not often) you'd be caught off-guard in too-high a gear and have to downshift for power.
It's definitely a laziness thing, but you appreciate always having power on tap at all areas of the rev range. Bike too twitchy in the tight stuff? Just upshift... and there's still power in the next gear despite the wide-ratio gearing - you're not stuck finding power between gears. I know a lot of people dial down the power on big adventure bikes when they're in the dirt or in the rain using "off-road" or "rain" mode, but I've been riding long enough that I can tame my right wrist to deliver appropriate power when and if I need it. I also turn off traction control too because it's too intrusive. I like having total control of the bike's full capabilities.
135hp is still not in ludicrous-horsepower territory, seeing how it has to move a 550lb motorcycle. The power-to-weight ratio is still in-line with most mid-weight adv bikes. I don't know how I'd feel about 170hp in the dirt though. Maybe I'd eat my words and select "off-road" power mode on the 1290?
Yes, they do offer a middle-weight GS. It's the F800-F850-and-now-F900GS which *was* the more dirt-oriented offering: 21" front, taller suspension than the 1200/1250GS. The original F800GS was revolutionary. At 450lbs, it was a *true* middle-weight bike with very capable off-road chops. It was all everyone was talking about at the time as a reaction to how pudgy and un-off-roadworthy the R1200GS got.
However, with the introduction of Euro 5 regs, BMW got lazy about trying to keep weight down, since most people buy GSes to park in front of Starbucks, instead of actually using them as marketed. This new breed of customers didn't mind at all that this "middle-weight" F850GS's weight ballooned to over the original weight of the R1200GS. For original fans of the F800GS, that was a real WTF moment and cemented BMW's "lost-the-plot" reputation.
BMW owns the Starbucks ADV crowd. They sell more "adventure" bikes to pavement pounders than any other brand.
However, they have lost a lot of ground in the off-road market, who sees them for what they've become: a street-biased bike brand which is lazily coasting on Long Way Round credentials established two decades ago. Any off-road group ride I do is dominated by 690s, 890s and T7s. People who ride dirt a lot know what's good and what's not, and the newer GSs have become not good for dirt.
I am hoping that with the R13GS, BMW will split streams with this platform: Recapture some of their original customers by offering a light-weight bike with good suspension and a decent 21/18 wheel offering for the dirt enthusiasts. And still be able to pack a pavement pounder with all the radar-assisted adaptive cruise control, lane departure assist, electronic aids, etc. so the Starbucks rider can just sit on the passenger seat and watch Netflix videos on the 13" TFT screen in front of them and control their motorcycle with buttons like a PS5 controller.
