How naval use made the Union Jack name stick

Why is the British Union Flag commonly called the Union Jack?

Its close association with Royal Navy use at the jack staff made Union Jack the familiar popular name, even though Union Flag is the formal term.

A jack is flown at a ship's bow, and the combined British flag gave the Royal Navy a visible way to show national allegiance. Naval use was especially prominent while army units did not all carry it. That association made Union Jack the familiar popular name. The distinction drawn here treats Union Flag as the technical term and Union Jack as a name embedded in Britain's naval culture.