It was a kind of nationalist affectation, to proclaim the ancestral, pre-Babylonian-exile, Israelite origins of the newly-independent Hasmonean state—a bit like the motto of the British Royal Family (Dieu et mon droit – ‘God and my right’) is in French, harking back to its Norman origins.
Read more: Q&A: Why do Judean coins minted during the Hasmonean dynasty or the first Jewish-Roman war contain text in paleo-Hebrew script?Modern Israel, by the way, does much the same: the Old Hebrew script is used on the modern sheqel coin (bottom left):
The difference is, in Hasmonean times, people knew what the Old Hebrew text said, whereas 99.9% of modern Israelis haven’t a clue: most people assume it says ‘sheqel’, but in fact, it spells Yehud, which ironically is not Hebrew, but the Persian name for its Judean province, dating back to the sixth century BCE, when Persia had just conquered the Babylonian empire, and allowed all exiled nations (the Judeans included) to return to their ancestral homes.
Notes by Autumn Light 