We truly do not know the reasoning of why we did not shoot it down ahead of time.
Maybe we wanted to see the balloon's maneuverability, or where China felt it was important to go over in the USA....letting us know more about what they know about us and our important sights, or what spying capabilities it could render....
Maybe our spy balloons were flying over China right at the same time and we didn't shoot theirs down cuz we didn't want them to shoot ours down?
Basically, what I am suggesting is that....
There is always more behind the scenes than meets the general public's eye, that we are not told about.... there wouldn't be national security classified stuff, if we knew or could know, everything!