Because someone may make a mod that runs at 24 FPS, which would use a frameskip of 2, and a skip_delta_factor of somewhere between 1/2 and 1. It's the delta factor for things that frameskip, and frameskip is used so that the closest appropriate delta factor is never further than 0.75 to 1.5x; and, for frametimes that are multiple of 1/60, it's the same -- perfect framerate compensation.

It's more "obvious" for the switch to set them like this. It might look like everything would use 1/2, but a closer examination of how frameskipping works in comparison to delta implies that it doesn't.