It's because photoresist is sensitive to deep blue out to UV so you want to avoid that kind of light as wafers with photoresist applied but not-yet-exposed are

It's because photoresist is sensitive to deep blue out to UV so you want to avoid that kind of light as wafers with photoresist applied but not-yet-exposed are running around in their robotic FOUPs through the facility. It's exactly analogous to red safe lights used for film print photography.