An issue a lot of negative space causes is that it reduces subject scale and subject visual weight in the frame.
Why have all that negative space compete with the viewer's eye for attention?
As mentioned, having the subject facing out of the frame is an issue because we tend to look where others are looking, and the viewer's eye is immediately directed out of the frame.
People have been doing visual art for a couple of thousand years now, and in that time have come to understand that some techniques (composition, lighting) work better than others.
By understanding the whys of long established guidelines of visual art composition, we don't have to redo the trial and error discovery process those before us have already done.
Occasionally, we can make a very successful image by applying the visual art composition guidelines in new ways.
Composition (visual arts) - Wikipedia, the free encyclopedia