(data from open sources)
B-2: we only have 20 of them, can carry 80x500LB or 16x2000LB weapons; in addition, much of the B-2 is (by necessity) very highly classified which makes it difficult to integrate into a package. In addition, the stealth capability, critical technology, and high cost makes them a national asset so the powers that be are necessarily reluctant to place it at risk/wear it out in day-to-day ops.
B-1: dropped ~30-40% of the JDAMs in the sandbox despite flying around 1% of the sorties, radar and sniper pod for targeting, can carry 84x500LB or 24X2000LB class weapons, loiter time measured in hours, excellent at low level as well as high speed (quicker you can get from the CAS orbit to troops in contact the better). Auto terrain following radar lets the crew fly low level in 100% IMC.
B-52: the nuclear procedures incidents over the past several years were partially due to the continual conventional deployment taskings eroding the ability of the force to focus on nuclear ops. Boomer crews are dedicated to a nuclear mission, for example, not taking several weeks off every few months to drive riverine patrol boats. In addition, as the referenced baseops thread states, the BUFF is getting long in the tooth as well. Fewer sensors for the dynamic targeting necessary for CAS. Slightly less payload (70K vs 75K for the Bone).
All 3 airplanes have much greater endurance than fighters - gas delivered by a tanker is much more expensive than gas pumped on the ground; in addition, the greater weapon capacity allows a greater time on station (24 vs 3-4 JDAM max for a fighter/84 unguided MK82 vs ~28 MK82 for A-6E vs ~6-9 for a fighter). The extra sensors and crew allow better mission management for some tasks as well.
