The camera is focused on the background so the foreground is blurry enough that the tritium dots didn't come into focus. Also, because of the way cameras see/meter light -- which is much different than the way our eyes & brain manage the same task -- the glow from the tritium got lost. That's exactly why the picture worked so well, in fact. Even though they were really night sights, in the photo they were essentially all-black sights. As I was
taking the picture I thought I was going to get a photo of how the night sights were the only thing I could see, proving night sights were useful.
Same conclusion, different approach.