They had to attack multiple islands that held air strips in order to get the american bombers close enough to japan. Bombers in WWII had very short ranges.
The B-29 had a 1,600-mile combat radius, which I wouldn't characterize as 'very short'
Enola Gay launched from Tinian which was captured in August 1944. Many costly island assaults happened after that
Also attacking these islands that held air strips prevented Japanese planes from taking off and shooting down the bombers.
as if the fighters couldn't be based in Japan itself
And what duestroop said regarding timing of the readiness of Fatman.
Yes it would have delayed the end of the war some, but it's hard to argue that a few months wait wouldn't be worth missing out on iwo jima, okinawa, et al
a legitimate argument is that they couldn't be sure the bomb would work, and if they put the war on hold only to find it a failure, that would have been even more costly
but this argument is looking back with 20/20 hindsight. Given that we know the bomb would indeed work, holding off on some of those island assaults would have been the prudent course.