Random thing i heard: spider based PM are (much) easier to make than pedal based PM. Given that the whole point is accuracy & reliability / repeatability. Because pedals move more than a spider, and there are 2 pedals vs 1 spider.
So this probably explains why there seems to be more activity around spider based PM than pedal based ones.
I've posted here before on why i dont understand why pedal based aren't the norm, because of how versatile they are. I guess that's one reason.