Baha’u’llah’s family came from Nur, a district in the Iranian province of Mazandaran, the province in north Iran at the south end of the Caspian Sea. This province has high mountain peaks in the south descending to the northern coastal plain bordering the sea. Because of the dense sub-tropical jungles of the lower parts of the province (a stark contrast to the dry desert conditions in much of the rest of Iran), it was always difficult area for invaders to penetrate.
The Zoroastrian royalty and nobility fled to these parts after the Arab Islamic invasion of Iran in the seventh century and Baha’u’llah’s family are said to have been descended from the last Zoroastrian monarch of Iran. Even when the people of this area finally converted to Islam centuries after that invasion, they mainly converted to the Zaydi form of Shi’i Islam as distinct from the Sunni Islam of most of the rest of Iran. It was only when the Safavid monarchs imposed Twelver Shi’i Islam [those who believed in the succession of Imams after the passing of Muhammad] on the whole country that Mazandaran fell into line with the rest of Iran.
(Moojan Momen, 'Baha’u’llah: A Short Biography')