Because the seven kingdoms are in a constant state of regicide and war. No one is in charge long enough or has enough people to spare on a insignificant island that can't be farmed, has no natural resources, has no gold, and can be ignored over King's Landing. With King's Landing fairly self sufficient even in a siege, controlling the bay does not mean as much as it used to. The only benefit to Dragonstone is its control of the Bay but if no one needs to bring in supplies via sea, that's useless.
Previously it was ruled by Stannis, but after Robert was killed and the Lannister coup forced the war of the five kings, Stannis left it to try to take his slightly more rightful claim to the throne. No one saw the use of Dragonglass, so Dragonstone was not seen as particularly important.