User blog comment:Argonash11/Why can't Quarians have see-through visors?/@comment-730941-20100713144933

Probably as others have said, to retain an aura of mystery around this species. Having a see-through visor would kinda defeat the purpose of all the effort Bioware went to to make them all mysterious and enigmatic... As for in-universe reasons, yeah probably something to do with radiation or sunlight - or even cultural reasons, perhaps?