There are many other islands in southern Japan that seem large enough for B29 runways (the current runway is 2 miles). Why didn't the USA pick the island of least resistance? I'd assume Okinawa was the most fortified due to its size and local population.
The Allies weren't taking Okinawa for B-29 runways. They had plenty of those in the Mariana Islands. They wanted it as a base for the invasion of Japan, both for ships and shorter-ranged aircraft. Okinawa has harbours, and Kadena Air Base had already been built by the Japanese.
This required taking all of the Ryukyu Islands, and that's exactly what was done. The Battle of Okinawa is the famous part of this campaign, because the Japanese concentrated their resistance there, knowing that while they held the main island, the other islands would be of limited use. You can't set up mobile fleet bases while the enemy are still within artillery or small-boat attack range. The other islands in the group were taken comparatively easily, so the combats are not famous.
Source: Okinawa, 1945: Final Assault on the Empire, Simon Foster, 1996.
