The short, cynical answer is that the ones with the money see little profit in it. Another is that PD is becoming more of a problem than it once was and institutions move slowly.

About manganese in particular- One of the core factors leading to PD is fetal exposure to a bacterial toxin called LPS which makes the immune system's microglia hypersensitive to future exposures. Manganese multiplies the effect. Even more interesting is the finding that the same toxin can trigger metal allergies! Goodness only knows what that does in the brain but PD is one choice.

Another possible source of exposure to manganese and even pesticides is the bathroom shower! Inhaled steam can carry contaminants into the system. That is a possible explanation of the observation that living in a home with well water seems to be a risk factor.