Short answer: I have no idea.
Longer answer: Really, I have no idea. And as far as I can tell, neither does anyone else, beyond speculation.
But the difference in susceptibility is real: here is one study performed at Kaiser Permanente, showing that men have almost double the risk of developing Parkinson’s:
Other studies seem to have found a somewhat smaller gap, but still significant (~50%). Race/ethnicity appears to play a smaller role, but possibly some significant differences.