Probably because , at one point in time, that was the prodominint style of wrench, so he made the kind of wrench that people were using....
There was probably a bit of style involved tho, it would have been simpler to make 2 engineers type wrenches with straight handles than one S shaped wrench, but not terribly so, with a forge it is not all that hard to bend an S shape in something...
The hard part is making the openings, which would have required a fair amount of tedious filing to get them to the precice size. The real secret of blacksmithing wasn't the forge and hammer, it was the file, with a good file you can make anything ; P