First, you can add any site you want manually. Over at the right end of the row where it says Top Sites, hover the mouse to get a ••• menu, then use the Add command.

But why isn't it appearing automatically? Hmm...

Do you always load the CNN home page on each visit? If each page you load has a different URL, they don't get added together to increase the frequency of visits, it needs to be the same URL.

If you think it might be on the list of blocked sites, you can clear it. Unfortunately, this may bring back sites you want to stay blocked (I don't know how to do it selectively).

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste blocked and pause while the list is filtered

(3) If the browser.newtabpage.blocked preference is bolded and "modified", you can use right-click > Reset to clear the list

(I don't know how to decode the sites in the list to be more selective, unfortunately.)

Here's a screenshot of what @jscher2000 is referring to:

Also, check that CNN is not blocked in Options -> Privacy & Security -> Cookies & Site Data -> Manage Permissions

jpbradley2001 said

Other sites which I have never visited fill out the section.

Maybe you'd like to do what I have done : Hover over the top right side of those "unwanted" sites - three dots will appear. Click on those three dots and then on "Edit" - replace the URL (and name) with one of a site that you frequently use or don't mind being there.