Or do they really like the gimmicky extras that Opera offers that much?
This. I started using Opera long before Chrome even existed. A basic thing: Hold the right-mouse button and click the left. It takes you back to the previous page. Chrome still doesn't do that.
It's the UI that I prefer, not the rendering engine.
The major reason that I use Opera is the built-in mail client. I have half a dozen email accounts and using multi-account login in Gmail is a bit of a pain. Having the email client built into the browser checks all my accounts for me (using IMAP) so I have one "Unread" tab open instead of 6 Gmail tabs.