Reasons for this:
1. Convention. In most other applications (especially data applications), Ctrl+D is duplicate. I often find myself deleting something instead of duplicating because of this.
2. Consistency. Pro has inconsistent support for duplication of items -- it is on some context menus, such as when copying layouts, but not all, like copying layers. Adding this to all context menus, and setting Ctrl+D to 'duplicate' by default would make this more consistent and save many clicks.
3. The Windows keyboard already has a delete key! How often does hardware have a dedicated key that matches up with an exact command in the software?! Why is a secondary, more onerous shortcut needed, especially when said shortcut is not only unconventional, but nearly the exact opposite of the convention?
related idea - Table editing - Ctrl+D to copy from cell above - Esri Community