Quote from "Of the Rings of Power and the Third Age":
"And in the days of Telemnar,the third and twentieth of the line of Meneldil, a plague came upon dark winds out of the east, and it smote the King and his children, and many of the people of of Gondor perished. Then the forts on the borders of Mordor were deserted, and Minas Ithil was emptied of its people; and evil entered again into the Black Land secretly ... And in the days of Eärnil they made their first stroke, and they came by night out of Mordor over the passes of the Mountains of Shadow, and took Minas Ithil for their abode; and they made it a place of such dread that none dared look upon it."

In my opinion this means that the chief reason why the Nazgul were able to capture Minas Ithil was that its status was very severely diminished after the plague. If anyone disagrees, please tell why this passage should not be interpreted so.

The diminished importance of MI would explain why Gondor didn't do its utmost to save the city, and the dread which filled the place would explain why it was not retaken.
-Foradan