Weird phrasing. All of them having something that is rare means it is not rare at all, right? And if all of them have it, it is not a “condition”, it is their genes, how they are.
Most people think of evolution as a string of improvements, however you can get negative traits in a population from a large enough genetic bottleneck or spread by being next to a very useful mutation. So it’s not necessarily an advantage for them, it could have just been bad luck.
Alternatively, there could have been some selective pressure for it as a means of improving night vision or something, we just don’t know.