I don't think there is a single reason Opera is unpopular. It just doesn't have a single stand-out feature people can associate it with.
Firefox's thing was openness, and an ecosystem of extensions. Chrome's is speed (and openness, and being sponsored by Google). Opera? It's fast, but not as fast as Chrome. Not open. No extensions. Built-in mail client? Gmail is better and easier.
When switching to a new browser, people have to put up with the inconvenience of some sites being broken, in return for some payoff. With Opera I can't see an appealing reason to do this.
Finally, I think Opera has given up on desktop users and concentrated all efforts on sales in the embedded market.
Firefox's thing was openness, and an ecosystem of extensions. Chrome's is speed (and openness, and being sponsored by Google). Opera? It's fast, but not as fast as Chrome. Not open. No extensions. Built-in mail client? Gmail is better and easier.
When switching to a new browser, people have to put up with the inconvenience of some sites being broken, in return for some payoff. With Opera I can't see an appealing reason to do this.
Finally, I think Opera has given up on desktop users and concentrated all efforts on sales in the embedded market.