Why is the lock option not available for some notes?

From the article linked above:

”You can only lock notes on your device and notes in iCloud. You can't lock notes that have tags, PDFs, audio, video, Keynote, Pages, Numbers documents, or notes that use IMAP to sync with accounts, like Yahoo, Gmail, and Hotmail. You also can't password protect notes that you share with someone else. If you want to stop sharing a note, open the note, tap the Add People button , then tap Manage Shared Note > Stop Sharing.”