Simple...it cost too much to manufacture. It WAS a beauty, though.
Cruise missiles/ICBMs did what the Valkyrie was supposed to do, and basically did the job better. That, and it's not easy to get delta wings to work (even now we need high-powered computers working while the planes are in-flight in order to get delta wings to work well). At least, that's what I've read, I don't know a whole lot about this subject.
The SA-5, and to some extent the Mig-25. About that time SAC figured out that high and fast was still vulnerable to missles and everybody in the whole could see you up there. They went with moving the B-52s down to the deck (well about 500 feet) and bought the B-1, which was designed from the outset to fly very low, like around 200 feet at high subsonic speeds. Now, with stealth, we are back to flying high again, but no need to fly all that fast, because if they can't see you, then can't catch you.
