The U.S. Air Force is having a hard time keeping its 67 B-1B bombers ready for action. The availability rate (aircraft you can send into action) is about 51 percent. Five years ago it was 56 percent. The B-1Bs are used to drop smart bombs in Iraq and Afghanistan. They are particularly popular in Afghanistan, because you can put one in the air, and it can cover the entire country. While the B-1B is twice as expensive to operate (per hour in the air) than the B-52, the B-1B can more quickly move to a new target over Afghanistan. Last month, B-1Bs flew 84 combat sorties. However, most of the time B-1Bs spend in the air (about 31 hours a month per aircraft) is for training. Combat sorties average about 11 hours, while training sorties tend to be shorter.
B-52s are not only cheaper to maintain, they have a higher availability rate (65 percent.) As a result, the air force wants to keep 76 B-52s in service (despite a Congressional mandate to reduce that number to 56.) With the development of GPS guided bombs (JDAM), heavy bombers have become the most cost-effective way to deliver support to ground forces. The B-52 is the cheapest American heavy bomber to operate.
In the last fifty years, the air force has developed six heavy bombers (the 240 ton B-52 in 1955, the 74 ton B-58 in 1960, the 47 ton FB-111 in 1969, the 260 ton B-70 in the 1960s, the 236 ton B-1 in 1985, and the 181 ton B-2 in 1992.) All of these were developed primarily to deliver nuclear weapons (bombs or missiles), but have proved more useful dropping non-nuclear bombs. Only the B-70 was cancelled before being deployed.
The well maintained B-52s are quite sturdy and have, on average, only 16,000 flying hours on them. The air force estimates that the B-52s won't become un-maintainable until they reach 28,000 flight hours. The B-1 and B-2 were meant to provide a high tech replacement for the B-52, but the end of the Cold War made that impractical. The kinds of anti-aircraft threats the B-1 and B-2 were designed to deal with never materialized. This left the B-52 as the most cost effective way to deliver bombs. The B-1s and B-2s are getting some of the same weapons carrying and communications upgrades as the B-52, if only because these more modern aircraft provide an expensive backup for the B-52.
The B-1B and B-2 are more expensive to operate because they haul around a lot of gear that is not needed for the current counter-terror operations. The B-1B can travel at high speed and very low altitude, to evade enemy air defenses. The B-2 is very difficult to detect on radar, but this ability is achieved with some expensive to maintain design features. Back in the 1950s, when the B-52 was designed, air warfare was a lot simpler, and so was the BUFF (Big, Ugly, Fat Fella, as the B-52 has long been known.) There are still potential enemies out there with Cold War grade air defenses, and the B-1s and B-2s are maintained to deal with that eventuality.