Because it's neither bundled with any operating system (like IE or Safari) nor can they afford to do a lot of advertising (like Chrome).

Firefox got popular because many sites advertised it as a 'better browser than IE' for free, I guess.

I tried to explain that above. Opera's revenue, and therefore their development and marketing and sales strategy, is primarily in the mobile market, where it has been "picked up" quite successfully. They haven't focused as much on competing for usage share in the desktop browser market. (And even so, 2% of all computer users in the world is not exactly a failure; it's a very large number!)

It's like a mirror image of Firefox , which focused almost exclusively (and successfully) on the desktop, but has not been picked up on mobile because it has not focused as much on development or strategy there.

Simple, back when the browser shake-up was going on (early 2000s) Opera was a paid browser, or a free browser with banner advertisements. It was also Windows only for a long time. I don't remember exactly when they came to their senses and admitted that no one pays for a web browser, but by that point it was too late.