It all boils down to logistics. For B-29 raids from China, every ton of bombs dropped, every pound of fuel burned and every single spare part used had to be flown in across the Himalayas - there wasn't a land route available, due to the Japanese occupation of Burma. In turn, the air effort required to support the B-29 raids required it's own massive logistical effort. B-29 raids from China could never reach the scale of those from islands in the Pacific. It was considerably easier to ship supplies, bombs and fuel to a Pacific island, given the vast amount of shipping available to the US. I'd also point out that the majority of B-29s flew from the Marianas islands. These were a considerable distance from Okinawa, which was captured to provide air support for a possible invasion of Japan.

To clarify what /u/thefourthmaninaboat said, the campaign against the Ryukyus and Okinawa in particular was not to support B-29 efforts against Japan; rather, it was to serve as a staging post for the invasion of Kyushu, to take place starting Nov. 1.

To quote the U.S. Army's 1948 history of the Okinawa campaign:

"The Ryukyus were within medium bomber range of Japan, and it was estimated that 780 bombers, together with the necessary number of fighters, could be based there. An advanced fleet anchorage was available in Okinawa. From these airfields and naval bases American air and naval forces could attack the main islands of Japan and, by intensified sea and air blockade, sever them from the Japanese conquests to the south. The captured bases could also be used to support further operations in the regions bordering on the East China Sea. Finally, the conquest of the Ryukyus would provide adequate supporting positions for the invasion of Kyushu and, subsequently, Honshu, the industrial heart of Japan."

Admiral Ernest King's Third Report to the Secretary of the Navy, dated Dec. 3, 1945, contains a similar item:

"The occupation of Iwo Jima had advanced this line to within 640 miles of Tokyo at the eastern end. The next step directed by the Joint Chiefs of Staff as to secure a position in the Nansei Shoto chain, which extends in a shallow loop from Kyushu, the southernmost of the main Japanese islands, down to Japanese-held Formosa. Okinawa, the largest and most populous island in this chain, offered numerous sites for airfields from which almost any type of plane could reach industrial Kyushu only 350 miles distant, and attack the enemy's communications to Korea, to the Chinese mainland, and to the Indo-China and Singapore areas. Since Okinawa also contained several excellent naval anchorages, it was chosen as the objective; the Operations against it followed immediately on those for the capture of Iwo Jima."

Capturing Okinawa was not important for the B-29s, it was important for the A-20s, A-26s, and P-47s that could act as scrub brushes over southern Japan, hitting the small, mobile targets that B-29 raids would pass up. Raids by aircraft carrier task forces could (and did) do the same, but they lacked the staying power that land-based aircraft from Okinawa offered. Furthermore, Okinawa would serve the same role in Operation Olympic, the invasion of Kyushu, as southern England did in the invasion of France. It would be a forward staging base, from which invasion craft would muster and gather before X-Day.