Why Pick Hypersonic Vehicles Over Ballistic Missiles?
Intermediate Range Ballistic Missiles (range <3,500-5,500 kms>) and Inter-Continental Ballistic Missiles (range > 5,500 kms) usually rise to a very high altitude (1000-1200 kms). After the various stages burn out, the missile, pulled by gravity, falls on a prescribed course (unless it is a Manoeuvrable Re-Entry Vehicle).
An August 2017 assessment by the Nuclear Threat Initiative premises that the President of the US has just 2-4 minutes to render a decision on intercepting incoming ballistic missiles fired by China/Russia and launch its own missiles against the latter. In case of contiguous nations (example: India-Pakistan; India-China), this timeline would get further compressed.
The detection-interception time in case of HCMs and HGVs, travelling at speeds between Mach-5 and Mach-20, would be far less. Besides, HGVs do not rise above 100-110 kms altitude; HCMs fly even lower, at about 20-30 kms altitude. Their flatter trajectories vis-à-vis ICBMs, along with the Earth’s curvature, further complicates the dynamics of detecting-intercepting.