Any Deviant artist can simply favorite as much art as they can, but to comment on every upload can be a difficult task— it varies with the number of how many artists they are watching, the time spend to brainstorm, type, and proofread the comment. Some might even pass yours and leave a fave behind, almost like dine and dash. I always believe that they either never has enough time to type on your art, or even at all. Some would do the one-word comment, but that is not genuine. While they forget yours they might move on with the other artists that are watching.
Or maybe some artists see their favorite fan art character on your gallery and notice more of their favorite fan art, so they watch you for more. But you keep on drawing your art like normal, and after uploading a few new pieces you seem to notice that your watchers’ grow less interested. Remember what I mentioned? Sometimes watch you solely for the fan-art you drew (pretentious bastards is what I call them). But I do believe they are not being true to themselves when one watches a gallery.
While they don’t have the time or doesn't please their needs, I try not to care if they are there watching my art anymore.
Remember, never get discouraged if nobody is watching you.