The End: Cancellation and the Dustbin of History
Three reasons.
One, the intercontinental Ballistic Missile (ICBM) became a tested and reliable system to deliver nuclear warheads. It was likewise immune to any defense system extant at the time, and speedily reached their targets.
Two, the weapon system was considered too provocative. If it flew over any allied countries during its low altitude flight, such as those of European allies in a scenario where the Warsaw Pact invades Western Europe. Anybody beneath the vehicle would be killed outright by the sonic boom, and buildings would be damaged. While Ted Merkle argued that the fallout produced by the reactor would be insufficient to be cause for serious concern, there is an argument to be made that, above a certain threshold, any amount of hazardous ionizing radiation is something to worry about.
Three, the weapon had no counter imaginable to planners at the time. Even to this day, hypersonic weaponry is incredibly difficult to detect and counter because these weapons are simply moving so low and so fast. US policymakers were worried that if they deployed a weapon like the SLAM, they would force the Soviet Union to create a similar device to maintain deterrence, leaving the US vulnerable to an unstoppable, devastating weapon.
The project's assets were quickly reassigned or mothballed. The mighty Tory II C ramjet was sealed, still fully assembled, in the remote control disassembly building. One marvel of technology became a tomb for another, and thankfully, we never had to find out what it was like to live in a world with air-breathing nuclear ramjets.