Tehran emerged as Iran's largest city because it was designated as Iran's capital in 1795. This is because of Tehran's geography, which sits in an excellent defensive position. Sitting in a valley surrounded by high mountains, there is only one entrance by which a substantially large army (large enough to defeat the defending forces) could enter the city. This allowed for an easy defense in the older days of warfare. With the threat of British and Russian imperialism, the Qajar kings sought out the safest place in the country.

Tehran's defensive value deteriorated with the development of airplane warfare and missiles, but it still stands as a bullwark against a ground invasion.

Or for that matter, why not cities along the Persian Gulf? Too far from the main population of Persians?