Why was Okinawa the last step in the Allies' island-hopping campaign?

Okinawa was one of the most heavily fortified and strategically located islands in the the island-hopping campaign. The Americans planned to use Okinawa as a launching base for air attacks and eventually an invasion into the larger islands of Japan. For these reasons, it made sense to make it the final island in the island-hopping campaign, after which the campaign would shift into an invasion into the heart of Japan.

This larger-scale invasion never occurred, however, as Truman elected to end the war by dropping two atomic bombs on Japan, rather than lose many more men and resources in a long and bloody invasion.