Hypersonic on a ballistic trajectory translates into a half hour or so of radar tracking time.
Hypersonic at a few 10s of thousands of feet above ground level means that an incoming warhead shows up on radar over the horizon with only a few minutes or seconds before impact.
Not much time to verity it’s really incoming, decide to activate interceptors, get a valid targeting solution, and fire away.
Like it or not MAD was a reason why the World haven’t seen another war and more bloody than two previous combined in 20th century.
It is somehow sad to see it flying out of window now.