You can only refer to something as unlikely based on some prior expectation. It was unlikely to be successful because someone deemed that to be the most probable outcome. The person who deemed it to be unlikely, and their reasons for doing so, are supremely important.

Maybe the film didn't have the budget of a major studio, so it was assumed it would struggle to find an audience.

Maybe the author didn't go with a reputed publishing house and instead self-published their work online, and without distribution, it was assumed the work would not be read by many people.

Maybe the politician didn't stand a chance against the incumbent because they had integrity and not a lot of wealthy stakeholders.

Critics will say what they think about a piece of art prior to its release, and they'll make judgment calls about how likely a work is to achieve success. When something they deemed a probable failure achieves success, they call it "unlikely" as a means of saving face. (Who woulda thunk?)

Maybe the critics are usually right, and their devised methods for computing probabilities of success are calibrated pretty well. Maybe some successes are actually unlikely when compared to some arbitrary standard set forth by a critic.

I remain cynical. I think it takes guts to create the unlikely. And now, more than ever, markets are willing to make the unlikely a success.