Maybe they are astounded to the point of speechlessness. :)
Commenting takes thought, time, effort.
It is wonderful to get comments, but when I get lots of views, I am equally happy.
Comments are wonderful and deserve your gratitude, but you can't necessarily expect them.
Many people don't feel comfortable commenting, or think their comments don't add anything significant. Some are just very busy.
I have come to some hubs with a lot of comments and don't feel I have anything to add. I have been to a few that need no comment.
Concentrate on what you are writing and forget about other people; You can control what you offer, you can't control the response of others.