This is a theme I have touched on, arguably harped on, in the past.
Here I mean Popular in the sense of "likeable" not in the sense of "everyone knows about it and likes it" -- the difference between the two is marketing, not photography, anyways.
Popular isn't the same as Good. Good isn't the same as Popular. Good also is not the opposite of Popular -- the fact that nobody likes your work is not evidence that your work is Good. An even more common, but still wrong, idea is that Good has nothing to do with Popular.
Good overlaps with Popular in the following way: In order to be Good a piece has to be able to affect most of its viewers. This implies, obviously, that most of its viewers need to be willing to look at it for long enough to be affected. A Popular photograph is one most people enjoy looking at (by definition).
An un-Popular photograph might still be visually arresting enough to be effective, but it's less likely. One will naturally tend to be dismissive of a photograph one dislikes, but which is aggressive enough to hold one's gaze. A photograph which is Good but not Popular has successfully overcome obstacles in its journey to affecting the viewer.
Good photographs tend, therefore, to be Popular. There are fewer obstacles to effectiveness.
Of course, there are second order effects: Famous Good Photographs tend to garner Popularity by virtue of their fame, and tend also to direct the public's appreciation of which should be Popular. This feedback loop draws Good photographs gradually into the category of Popular, increasing the overlap between the categories.