Okinawa was going to be used, but it was only able receive heavy planes right before the war ended. Remember: The battle for Okinawa ended just a few months before the war ended.
The US planed to base the 8th airforce out of Okinawa, with the principle goal of having them support the invasion of Japan. As part of this B-29 units such as the 22d, 333d and 346th Bombardment Groups were assigned to Okinawa. However the first B-29 didn't arrive on the island until early August 1945, and these units were not able to enter combat before the end of the war.

Iwo Jima's biggest problem was that it was tiny. Only one of the two airfields could really support B-29s (South field was long enough for emergency landings but not for regular use with combat loaded planes). Central Field (aka 'the north field') could theoretically operate B-29s but didn't have a lot of space left over once all the fighters the 20th airforce wanted to station there were accounted for.