The hexagram is one of the easiest figures to draw. Two overlapping equilateral triangles fall out of any compass-and-straightedge exercise, and the shape turns up as ornament in Mesopotamian, Indian, Islamic and Christian contexts with no shared meaning between them. This matters, because it means the mere appearance of a hexagram somewhere old is not evidence of anything Jewish.
It does appear in Jewish settings in antiquity. A hexagram is carved on a stone from a late Roman synagogue in the Galilee; one occurs in the decoration of the Leningrad Codex of 1008, the oldest complete Hebrew Bible manuscript. What is missing in every one of these cases is any indication that it was understood as a sign of Jewish identity. It sits among rosettes, knots and interlace as one pattern among many.
The evidence for it as an emblem begins much later and in a particular place. In 1354 the Bohemian king Charles IV granted the Jewish community of Prague the right to a flag, and the flag carried a hexagram. Prague Hebrew printing used it on title pages from 1512. From Prague it spread through the Jewish communities of central and then eastern Europe over the following centuries.
Gershom Scholem gave the standard account of why. Writing on the history of the symbol, he argued that its rise as a general Jewish emblem belongs to the era of emancipation, when Jewish communities entering European civic life needed a single recognisable sign of the kind Christianity already had in the cross. The hexagram was available, unencumbered by doctrinal content, and easy to reproduce. Its adoption was, on this reading, a response to a modern situation rather than the recovery of an ancient one.