The most often reason is misconfiguration in your WordPress. Check the following:
Turning on Comments for a Single Post or Page
On the post or page you wish to add comments to, find the “Discussion” box, and check “Allow Comments.” (If you do not see the “Discussion” box on the Edit Page, click “Screen Options” in the upper right corner of the browser window. Make sure the box next to “Discussion” is checked.) Save your changes by clicking “Publish”, “Save Draft” or “Update.
Turning on Comments from the Posts or Pages Screen Turning on Comments from the Posts or Pages Screen
Navigate to the Posts/Pages screen. In the list of pages or posts, find the one you want and hover your cursor over the title of the post. You will see several links appear underneath the title. Click “Quick Edit” and check “Allow Comments.” Click “Update” to turn comments on for that post.
Turning on Comments for Multiple Posts or Pages Turning on Comments for Multiple Posts or Pages
From the Posts/Pages screen, check the boxes next to the posts or Pages on which you want to enable comments. Select “Edit” from the “Bulk Edit” Dropdown box and click Apply. Then, select “Allow” next in the Comments dropdown box and finish by clicking “update.”