This got me thinking
What makes Russian use of Kinzhal special?
Technically speaking most Ballistic Missiles are hypersonic then what makes Kinzhal special besides being air-launch based ballistic missile?
Whats special about hypersonics and distinctly different from ballistics is the flight path.
Ballistic missiles fly high and gain speed from gravity assisted dive to reach hypersonic terminal speeds. That makes them easier to detect at extended ranges and gives you more options to intercept them since their flight paths are fairly fixed and predictable.
Hypersonics can fly at cruise missile height at hypersonic speeds. That massively reduced the detection window and their speed and manoeuvrability makes intercept far far more challenging.
The plasma sheaf that forms around the missile can also work to further reduce RCS and may even make hypersonics far more resistant to direct energy laser weapons.
Currently there are defences that can at least have a reasonable chance of intercepting ballistic missiles, but no weapon can claim to be able to intercept an hypersonic other than by sheer blind luck.