Sometimes it's because they were sleeping. But usually it's because I am afraid of eyes. They just seem very hard to draw.
And they are so easy to get wrong. If you draw the right eye when they happen to be looking left, and the left eye when they happen to be looking right, they will look cross-eyed.
If you see any future sketches with the eyes closed, hopefully it will be because the subject actually had them closed.
Sketch a Day
I try to complete at least one sketch each day.