As far as I can see the StVZO 6V/3W basic specification has been established and then enforced in the years to guarantee sufficient light using a traditional 6V/3W (12 Ohm) filament incandescent light (or its electrical equivalent represented by a 2.4W-15 Ohm front light in parallel with a 0.6W-60 Ohm rear light). But today even halogen lamps have been replaced by more modern and more efficient LED lights capable indeed to deliver more lumen/lux per watt, and that’s why nowadays on the market do exist 1.5 W StVZO hub dynamos.
But today, IMO, dynamos are becoming pretty much used not only for lightening but also for charging usb devices … and in this case 3W might not be enough!!!
To produce more powerful dynamos involves of course technical implications such as weight, dragging and quite possibly also output voltage, to the point that some safety hazard issue might arise.
So what’s the real reason for not producing dynamos capable to deliver more than 3W ?