IMAP usually means that mail is stored on the provider server even if one can download and delete. Furthermore POP3 is a trivial protocol that could be operated via telnet before everybody went TLS.
The real reason I'm still using POP3 is that I'm using the mailboxes that are bundled with my domains. One on the registrars announced IMAP support a few days ago. All the others are still on POP3 probably because POP3 servers have been available since forever.