This touches on most of the reasoning but leaves out the most obvious thing: the B-52 is as close to the "ideal" for a subsonic bomb truck that we've been able to come up with.
Look, we need different types and numbers of aircraft for any given military engagement. They should be:
1. A small, elite force of the best stealth and precision-targeting weapons we can muster as a nation, flown by the most rigidly-trained men and women our nation has to offer. These forces lead the initial attack, destroying command and control facilities, air defenses, air bases, and logistics. Given our huge lead in these areas of technology over our hypothetical foes, these missions should have a very high success rate and low attrition rate.
2. Once the enemy air force and air defenses have been beaten down, the value of stealth is greatly diminished. At this point we're far better served by cheaper, less maintenance-intensive forces like a B-52 or A-10. Their ruggedness and numbers make them ideal for this lower-threat environment where stealthy options would be too expensive to maintain and too valuable to lose in a protracted fight.
3. When all major targets are bombed into oblivion by the forces in item #2, you need to keep a constant presence around to deter regrowth. But things like B-52's are expensive to keep flying around waiting on something to bomb. Far better to spend money on armed drones which can orbit for a full day, out of sight, waiting to deliver a "bolt from the blue" when a bad guy steps outside for a piss. All the destructive power of a precision-guided munition married to the omnipresent terror of a sniper in an unknown and unreachable location.