The problem with invading the Kuriles as the US for the purposes of a strategic bombing campaign are several. Firstly, the Kuriles island chain is relatively small, and airfields suitable for fighter deployment are not necessarily suitable for large bombers. Of areas that would be suitable, Paramushiru would probably be the most likely, given its size and the fact that there were already several small airfields there. These airfields would still need to be repaired and developed, however, which takes time, engineers, and construction equipment. All of this would naturally need to be hauled in, in addition to troops.

The difficulties in the Kuriles vs. the Marianas, however, are several times worse. Firstly, the weather conditions were much worse up north in the Kuriles, with snow and frost either grounding flights or causing maintenance damage to planes, which could be extremely time consuming, especially for large 4 engine bombers like the B-29. Secondly, the capture of the island would require naval superiority. Unlike the Marianas, the Kuriles were in range of Japanese air bases in Hokkaido. Japanese reinforcements could easily be dispatched via air or even barge. The US would need to disable those air fields to have any chance of securing an airbase, and to do so would require either already constructed air bases in range or a constant naval presence. This naval presence would further need maintenance and resupply, and the nearest major port facilities would either be in Alaska or Pearl Harbor. And all of this would be exposed to air raids from aircraft based in Hokkaido. Suppressing Hokkaido would further stress the US Navy and may potentially have resulted in local Japanese naval successes as overstretched Allied forces would become vulnerable.

In the case of the Marianas, while there were mutually supporting airbases, there was no easy way to ship in reinforcements or fly air missions from, say, the Philippines. With their island airbases under attack, the only way to stop the Americans from constructing their own air bases would be through naval bombardment, which simply could not happen with the presence of the US navy.