Well, the threat of the Soviet Union's combined arms armies driving through the Fulda Gap into western Europe doesn't exist anymore certainly, alongside with the need for an aircraft to keep tabs on such massive military movements. Today's immediate threats like, say, Iran, don't warrant such a high-spec recon aircraft, and China doesn't really have a big obvious military invasion plan with one vector of advance like the Soviets did.
The use as a space launch platform is a possibility, though U-2, M-55 and White Knight can also do that while being cheaper and less maintenance-intensive