Quite possibly because nobody can afford a Stahiwille torque wrench.
Seriously, both are well credentialed, well known and indeed well envied tool makers. How could you go wrong.
I have a preference from Stahlwille screwdrivers and spanners, 40 years after that preference was established I am still slowly adding to my collection, bit by bit. I have 5mm to 27mm ring/open ended spanners and 6 assorted phillips and blade screwdrivers......
We have had this discussion before in this group regarding torque wrenches, i have a preference for Warren & Brown "deflecting beam" type torque wrenches. Whilst I know they are a very good tool my main reason for owning them is that there is a local recalibration service available, which is important when you buy tools you intend to own for a lifetime.
Please note, I am not saying that deflecting beam type torque wrenches are in any way better than any other design, they are simply what I grew up with.