Which island has a ghost town as its de jure capital? Why does no one live there?

Montserrat. The capital is Plymouth.
There are no inhabitants due to the repeated eruptions from the Soufrière Hills volcano.

https://en.wikipedia.org/wiki/Plymouth,_Montserrat

https://en.wikipedia.org/wiki/Plymouth,_Montserrat#Volcanic_eruptions_and_abandonment

Because the Soufrière Hills volcano began erupting in the mid?1990s and continued with repeated, destructive eruptions. Plymouth was buried under ash and pyroclastic flows, forcing the entire population to evacuate. Montserrat moved its government functions north to Brades, but Plymouth remains the island's official capital on paper - a rare case of a legally designated capital that is completely abandoned.