Settings & keep-time · Keyboard shortcuts
Why is a shortcut missing from the list when I pick a Storage Studio hotkey?
Short answer: Another Storage Studio action is already using that combination, and each one can belong to only one action. Change or switch off the action holding it in Settings → Shortcuts to free it up.
Because another Storage Studio action is already using it. Each combination can belong to only one action, so a combination in use is hidden from the other actions' menus — that keeps the Apps grid and Junk Yard shortcuts from clashing.
To free one up, change or switch off the action that holds it in Settings → Shortcuts, and it reappears in the other menus.