In glaucoma why are proglastalins eye drops prescribed most frequently when eye color change and darkening skin are common side effects?

Because. Most efficacious (first line among meds), easiest dosing (once a day), limited side effects (generally well-tolerated), with dark eyes and dark skin, cosmetic changes less important.
Efficacy/simplicity . Prostaglandin analogues are some of the most effective anti glaucoma medication available to treat glaucoma. A single drop/day lowers iop 30-32% over 24 hours, while have a very insignificant systemic side effect profile. Unfortunately, several cosmetic side effects (lash growth, iris color change, peri ocular pigmentation, sulcus deformity). Basically they are used because of their great benefit.