Budget constraint and obsolescence. By the time it was withdrawn, the Russians had finally managed to get the capability to shoot it down. The MiG-31 with the

Budget constraint and obsolescence. By the time it was withdrawn, the Russians had finally managed to get the capability to shoot it down. The MiG-31 with the R-33/AA-9 missile could engage a blackbird. It interdicted soviet union overflights in the mid 80s. Modern Russian SAMs is probably also able to intercept it. If the ability to penetrate the targeted air defences is taken away, you're better off just flying a jetliner crammed full of sensors around the target country or use satellite. Or drones (since they're disposable-ish when they get shot down.) The Open Skies Treaty also allows mutual airborne surveillance. The SR-71 however, wins hands down on sexiness.