I think a big part of it is that after many years of everyone using bright, low contrast, default lighting, a lot of people are now experimenting with the environment settings in routes/sessions.
We tend to not notice the ones that look like a normal clear day, as although greater in contrast they still look like a fairly normal 'brightly lit' scene (which is correct, if you want a nice normal clear day
).
But we
do notice the darker, often much contrastier, as they aren't what we're as used to. But often these represent overcast or semi-overcast days. Personally I've taken a lot of pleasure lately in playing with dreary wet days, as these can offer some nice looks to the shots, especially with steam locos. But of course it's quite possible to overdo this, and to a large degree
this will depend on the screens of both the person taking the screenshot and the viewer. Contrast can be one of the biggest variations between screens, even the two screens on my system show a massive difference in brightness/contrast, and so on a 'low contrast' screen, the scene will look good. But take a screenshot and then put it on a more normal/accurate contrast screen, and it could look overdone.
Regards