1. Breaking from the Ballistic Missile Treaty.
2. Cost
It always made sense in that it was technically possible already many years ago (Sprint, Nike Hercules, Hawk TBM), it's a true defensive measure, and there has been a growing threat for a long-long time. Already in 1991 with the older version of the Patriot the real world practical application of this was put to test in Desert Shield / Storm when shooting down SCUD missiles.
The people that claimed they had some engineering / science argument against it, made no sense.
If anything, it was reckless and irresponsible (sorry for the cliche, but true in this case) not to pour needed resources into missile defense and slow roll it as we did for many years.
It’s like the old adage given to thatcher by ira; you have to be lucky every time, I only have to get lucky once. It was a victory for iran if only one missile got through. You will notice Israel hasn’t fired back.