Ah, that pesky thing known as "free will". We get to choose what we wear on our body and our head. I think one of the things that makes hi-viz effective is that it's not seen often. If everyone wore it, other drivers might become "used to seeing it" and it's effectiveness might be reduced. Sounds like we need a University study here to flesh out that idea.
I just look at my "wall of helmets" and decide what I feel like wearing each time I ride. It's usually something that I don't need to grab a step stool to reach.