At some point, a person who was working on the code for this section of WikiTree (probably accidentally) disabled right-click for cut/copy/paste in this textarea. If they disabled the cut/copy/paste events, the key combinations noted above would not work.
Categorize this question with WikiTree Tech to bring it to the attention of the proper group.
From a quick view of the page source, it appears that the problem might be in the CSS for class qa_form_tall_data or a related class.
[Edit: Categorize, not Tag.]
[Edit: might be, not is.]