Idk. The brain was (and really still is) very badly understood. But even back then, they already knew that nerves connected roughly in that area. Then a theory

Idk. The brain was (and really still is) very badly understood. But even back then, they already knew that nerves connected roughly in that area. Then a theory that posits a minimal topological mapping between the two makes sense. I can imagine the motor cortex evolving like that, or growing with the fetal development. So in the early days, this was a sensible model. It overstayed its welcome, as so many theories do. Many scientists still hold on to coarse brain regionality.