If you want to know why your final crop looks better, you have to understand how the eyes and brain work when looking at an image.
In your first posted horizontal shot, even when you try to focus on the eyes, the viewer's eyes keep going back and forth to the lines in the woman's sweater because eyes are attracted to contrast and are easily distracted. Why I suggested the crop is simply because I could feel my eyes wanting to go to her back instead of staying on the point of interest that is the faces in this case.
This is one thing one must not forget when composing either a shot or cropping in post. You must make sure you have a point of interest and that there is not too much contrasting elements that distracts from the main subject.