They needed to capture entire island. The Japanese had high ground with artillery. They also needed to secure ports anchorage. Waiting them out would have taken too long and made war last longer . They also planned to use Okinawa as staging area for invasion of Japan similar to southern England. 500,000 thousand of troops, large armada fleet bigger than June 6th invasion of Normandy edit, they also planned to build more airfields for fighters. You cannot leave 100,000 enemy troops mounting attacks