I would guess that the reason for this is mostly because Parksinson's itself is not that well understood.

It makes sense to say "yes it might work" because we know for sure the drug does xxxx and Parkinson's might have xxxx disease path and therefore cause xxxx which means xxxx symptom of Parkinson's might be due to xxxx which does respond to the drug

On the other hand it makes sense to say "no it might not work" because even though we know the drug does xxxx, Parkinson's might have alternative xxxx disease path and cause xxxx which means xxxx symptom of Parkinson's might be due to alternative xxxx which doesn't respond to the drug

I say it might work because either of these situations might be true

The top comment about it being too late for the actual cells is true, damage brain cells and connections can't be fixed but the damage can be prevented. I'm speaking on the prevention and/or slowing of disease progression