>>64065967https://en.wikipedia.org/wiki/Comparison_of_anti-ballistic_missile_systems
SM-3 IIA has a 21" motor and a max altitude of ~600 miles (all the way into the bottom end of MEO).
THAAD has a 13" motor and a max altitude of 90 miles (just below LEO).
That's a pretty big difference in delta-v. Think of IIA as exo-atmospheric, and THAAD as endo-atmospheric. That means SM-3 can hit mid-course targets, while THAAD can only hit terminal targets within a much smaller radius.
So, why not replace the THAAD with IIA? Well, look at the estimated prices: $12M for THAAD, vs. $28M for IIA (or $70M for GBI). You need more fuel to reach longer ranges. You need more thrust (and hence more fuel) to reach the greater speeds needed to get to those longer ranges before the inbounds can pass by. You need more structure and better materials to handle the extra speed and acceleration. It all snowballs.
And
>>64065990 is correct; there was so much NIMBY that Japan gave up and decided to put all their eggs in the naval basket; this has the advantage of being mobile, but of course the disadvantage of being far easier for the enemy to suppress or destroy. It does also have a bit of an advantage in that the ships can probably be deployed off Japan's east coast and still defend the west coast from ballistic missiles, making attempts to find or kill them more difficult for the PLA.