Wrong type of key for the application - Woodruff keys are for alignment, not driving.
If an alternative drive arrangement were required, then a spline would have been better. But that would have required the shaft the be larger diameter, which would have forced a larger diameter for the primary drive gear bearing that runs inboard of the drive on the same shaft (which may not be feasible). Plus cutting accurate splines & broaching the hub would have cost more.
The driving taper is compact, easy to make (i.e cheap ! ) & works well if assembled correctly.